On reading a book about the middle ages in Europe (simply "The Middle Ages" by Joseph Dahmus), I came accross an interesting point involving vassalage. It said that on certain occasions a vassal pledging military service might render service to more than one Lord. If a situation came where these two would fight, the vassal would resolve this issue by siding with the lord most likely to win. That in itself makes for an interesting discussion, but it's not what I wanted to know. My question is whether or not it was a common occurence for local lords in proximity to each other, although both serving the same king, to fight one another. Or, in contrast, is this something that did occur, but not as often is it might seem? How did Europe get out of a pitfall like that?
Edits:
In order to narrow the question down, what I really want to know is at what specific points in medieval feudal history (1000-1300) that this 'feuding' took place. And when I say war, I mean more serious conflicts between lords. This would be a situation where a fair amount of soldiers get involved. It seems reasonable to assume that mere land disputes at the local level would have been commonplace at a time of such decentralization (or maybe I'm wrong?). I should also point out that I mostly have Western Europe in mind; I want to narrow it down there before turning attention to other regions.
Disclaimer: I am very sick at the moment, so this may be a little bit rougher than my usual answers.
The period that comprised the Angevin Empire might be of particular interest to you. As Duke of Normandy, Count of Anjou, and Duke of Aquitaine by right of his wife, Henry II of England was not only a King in his own right, but also a vassal of the King of France. Henry, and later his sons, would regularly do homage to the Kings of France for their lands in France, despite their power rivaling or surpassing that of King Louis VII.
In 1159, Henry II invaded Toulouse on the grounds that his wife, Eleanor, had a claim to the county through her grandmother. However, the conflict was very swiftly halted when Louis VII showed up in Toulouse and married his sister to Count Raymond of Toulouse. Continuing the conflict with Louis in residence would have escalated the conflict to a full on Kingdom vs. Kingdom war, rather than Henry simply acting in his capacity as a vassal.
Another example of the vassal-lord-king relationship in action occurs in the Chronicle of Jordan Fantosme. Jordan was chronicling the revolt of 1173 in which Henry II's sons, and several of his vassals rebelled against him. Jordan tells us mostly about events in England, notably the invasion of the North by the King of Scotland, who was also a vassal of Henry's, as he held the Earldoms of Huntingdon and Northampton. Jordan also recorded the actions of rebels in the Southeast of England. In one particular incident, the Earl of Norfolk, Hugh Bigod, attempted to seize the City of Norwich. However, the citizens of Norwich declared that their allegiance was first to the King, and secondly to any local lord and repelled the Earl's advances. Any serious rebel activity in England crumbled soon after.
Similar conflicts occurred during the Anarchy (1135-1154) in England. Many English lords manipulated their allegiances for personal gain. Ranulf of Chester most notably changed sides between Stephen and Matilda nearly a dozen times, and spent quite a bit of the conflict fighting a private campaign against Robert de Beaumont, Earl of Leicester. How common it was during this period for lords to fight over land with little to no royal oversight is part of how this period gets its name. As a result, England was covered with 'adulterine' castles that had been constructed without royal consent. Henry II had these castles demolished as a symbolic return to order. However, it is important to note that England was significantly more centralized than France during the 12th Century, and so conflict among vassals did not occur as frequently in England.
The chronicler, Orderic Vitalis, a monk in the abbey of St. Evroul in Normandy, committed quite a bit of his writing to the exploits of local lords. Orderic took umbrage largely with Robert de Belleme, whose tactics in expanding his holdings and wealth often crossed the line of what was considered acceptable. He accuses Robert of crimes towards churches, the poor, and the innocent. However, Robert was in conflict with families that were primary donors to Orderic's monastic house, which gives Orderic a massive bias. Despite his biases, Orderic's writings give a modern reader a good notion of conflicts between vassals. Much of the conflicts involve traditional campaigns against the others lands, with smaller-scale skirmishes fought over sometimes-tiny pieces of land. If you're interested in direct vassal-on-vassal conflict, I would definitely suggest reading some Orderic. Unfortunately, the current translation (by Marjorie Chibnall) is currently out of print, and used copies of just one of the six volumes can run in the hundreds of dollars. However, you can sometimes find copies of the 1850s translation freely available online through public education resources like archive dot org.
Another source, which I think you'll like even more, is a popular Chanson de Geste that was probably written at some point during that 12th C. Raoul de Cambrai is a pseudo-historical poem about conflicts over the Country of Vermandois between two rival lords, the titular Raoul, and Bernier, a young man who grew up as an underling of Raoul's, but who eventually challenges his claim to Vermandois. While the poem claims to be a telling of historical events, it uses concepts and conventions that would have been much more familiar to the 12th C audience, and essentially functions as a guide to the expectations and conventions of a lord-vassal relationship and the conduct of vassals-on-vassal feuding. Getting the full text of Raoul is also difficult, but De Rei Militari actually has an outdated version in full posted HERE.
Sorry that this is a little bit word-vomit and not as organized as I would have liked. I definitely think Orderic and Raoul are good examples of what you're looking for. Vassal-on-vassal conflict was definitely common. Overlords sometimes intervened, but largely if it was in their own interest. Please reply if you have any more specific questions on this topic and I'd be happy to get a little bit further into research as I am able.