I watched the 3rd season of The Last Kingdom recently, and in the final episode there's a large battle where one army is ambushed by the other while it's on the march. Partway through the battle, two more armies join in, both of which were a surprise to the two armies already engaged.
Was this a common occurrence in the past? If not, how effective were scouts and pickets at relaying accurate information?
As I've mentioned in other threads, Wessex during the period in which The Last Kingdom is set was actually studded by a fairly extensive and sophisticated network of look-out posts and signalling beacons connected with garisson forces at local burh fortifications. This network was also then extended into Mercia and into recaptured areas of the Danelaw. This system was designed specifically to curtail the Viking strategy of mobile warfare and afforded the English both the early warning and the use of the rivers and road network to respond rapidly to Danish threats. Indeed, significant victories such as the 911 Battle of Tettenhall come about because the English are aware of where the Danes are and where they are heading, and are able to outmanoeuvre and surprise them.
In addition to the excellent answer provided by /u/BRIStoneman in the context of Anglo-Saxon England and the Viking incursions, I may be able to shed some light on the nature of military intelligence and some of the methods by which it could be obtained in the context of feudal western Europe during the High Middle Ages, and to some extent the Late Middle Ages.
First of all, I will approach your inquiry from the perspective of organisational stages: initiating the war, the strategy, and the tactics.
Secondly, I will mention the restrictions that decided to what degree feudal combatants were able to achieve strategic and tactical surprise before going over the options of military intelligence available to them, as the former dictates the the latter.
Lastly, examples and sources I use in the following are local to my area, the western Netherlands, but can in most contexts be applied to much of feudal western Europe due to parallels in feudalism, Christian values, and the perception of knightly honour.
The (high) nobility operated in a very personalised international social network in which nearly everyone was acquainted in some way, making war a personal affair and therefore predictable in its initiation in terms of underlying reasons and the aimed objectives.
From the High Middle Ages onward, the nobility of feudal western Europe was a well-connected clique of acquainted individuals with vassal-liege ties, marriage ties, or some form of relation through (short-term) coalition or (long-term) alliance. As such, war between feudal combatants was personal: not rarely it involved raising arms against a liege, a vassal or indeed family, either by blood or in-laws. This also meant that wars were generally preceded by conflict in other areas, such as arguments over trade tariffs, the ownership of certain fiefs or rights, or simply over rivalries of some kind or another, leading to diplomatic or economic action that only resulted in war if a suitable compromise could not be reached. As such, the onset of war was readily apparent, as was the reason to wage a war.
The cumbersome nature of the organisation and mobilisation of feudal forces as well as of the logistic apparatus supporting them meant that it was nigh impossible to achieve strategic surprise.
The military forces of feudal western Europe were structured on the local approximates of the levy system: the nobility and their retinues, and the common man, were summoned (the former by personal invitation of the liege, the latter by local representatives) and requested to converge on predetermined mustering locations where inspection of the assembled forces would take place. In the County of Holland the period of time allocated to reach these mustering locations was put at 14 days, a period of time that depended on the size of the territory. The levy itself could be fielded for 40 days (this was also true in other fiefdoms in the Holy Roman Empire, as well as in the Kingdom of England and the Kingdom of France), at which point a new levy had to be raised, in order to minimise the impact on the available workforce and therefore the economy. The combination of set periods of time between summoning and mustering, the troops assembling at predetermined locations, as well as the limited window in which military goals had to be realised, meant that there was no such thing as a speedily and/or covert build-up of forces. As a result, not only was the initiation of a war an obvious process, it also meant that the initial location of the enemy forces was a predictable one.
Furthermore, the logistics coupled with fielding a feudal army were very observable: rations and supplies were purchased in large quantities in the cities, and carts and barges were commandeered and chartered to transport these goods. This meant that any local merchant or inhabitant was aware of what transpired, and especially knowledge in the hands of the former quickly found its way to colleagues abroad, something I will touch upon later. This also meant that it was rather easy to conclude what the military goal of a campaign was, as the nature and amount of goods hinted towards the strategy employed (i.e. oats for horses in large quantities suggested use of cavalry), as well as the envisioned goal (i.e. material for siege engines implied the capture of a castle or urban centre), which is coincidentally also the reason why receipts of these orders and transactions are some of the most trustworthy primary sources we have on warfare in the High and Late Middle Ages (there is little to no embellishment, for instance).
The dependency on civil elements in the military meant that passive military intelligence represented a relatively large portion of the available intelligence in comparison to active military intelligence.
Not only was the common man an integral part of the military in the sense of forming the bulk of the levy, the logistic apparatus also heavily involved the population of urban centres. This meant that it was relatively easy to obtain reliable intelligence on enemy actions. As I mentioned before, the merchants were an accessible source of information, as they relied on an international trade network that was by nature effective in spreading precise information relatively quickly. The active gathering of intelligence happened in two roughly distinctive methods: bribing local representatives or the clergy, and the use of couriers (spies). Especially the use of couriers in the period is well documented, with their costs and payments represented in accounts, often anonymously in order to protect their identity. Between these sources of information, lieges and their councils had the opportunity to be well informed about the strategic and operational intentions of their enemy.
Relatively primitive methods of communication restricted the coordination of complex tactical manoeuvres, limiting the available options, and simplifying the tactical approach in the field.
The absence of a structural and dependable system of communication was a hindrance to tactical creativity, which was further enhanced during moments of friction (the appearance of phenomena that hinder plans, such as the weather or disease): a moonless night hampered vision in such a way that it was dangerous to manoeuvre military elements (i.e. two armies fielded by the Bishopric Utrecht failed to united on a night in October 1228, botching the planned offensive^(1)), so would fog or heavy rainfall. This simplification-by-necessity meant that the field of battle between feudal opponents was decidedly less a matter of surprise – which requires maximum communicative effectiveness – and more of cohesion – minimizing the needed communicative effectiveness. As a result, manoeuvring on the field of battle was relatively predictable.
There were of course exceptions to the rule, which only meant that tactical surprise was all the more devastating when it was achieved (purportedly the citizens of Utrecht thought they were surrounded by the Tartars rather than the West-Frisians in the dead of night in 1247^(2)), but by and large tactical success stories in this time period are occasions of non-feudal opponents, often using irregular forms of warfare, against feudal opponents using regular forms of warfare.
To conclude, the political and military realities of the period meant that information on strategic and operational intentions were relatively easily obtained, while information on the enemy tactics were more difficult to gather but oftentimes equally predictable. As such, feudal commanders in the High Middle Ages had the potential to be very well informed about an opposing force.
^(1 Quedam Narracio, Unknown Author, 1232)
^(2 Croniken van den Stichte van Utrecht ende van Hollant, Johannes Beke, 1371)