Yes this actually happens. It’s rare with such well documented persons but there was one “emperor” during the third century crisis who was “lost” to history until the 1930s when he was found on a random coin in the British museum.

Emperor Silbannacus is now believed to have reigned briefly during the Third Century Crisis. Now, it’s important to stress that we really don’t know much, if anything, about this guy so it’s impossible to give the kind of detailed response we like to see on this sub. However, I’ll link to an interesting article on how he was “discovered” below.

Regarding the part of your question on how “firm” the imperial succession was, the answer is not very. We are way before male primogeniture in terms of picking a successor emperor for most of Western Roman history. Augustus for example (who never actually called himself “emperor” by the way, he was the “first citizen”) probably wanted to leave the purple to Drusus and there is evidence he did not want Tiberius to succeed.

Basically, if you were popular with the army and happened to be in Rome when the emperor died, you had a pretty good chance of succeeding. However, there was an equally good chance of another general disagreeing with you on that and going for it themselves (for example the “year of the four emperors” AND later the “year of the five emperors”, as well as at various other points, hence our friend “emperor” Silbannacus of earlier). Essentially, the imperial succession came down to “might makes right”.

The emperor also was not a Monarch as we would understand from the Middle Ages. So called “co-emperors” were very common, particularly later into the empire. Marcus Aurelius for example ruled alongside Lucius Verus from 161-169 (Verus died) and later his son Commodus (yes, that one) from 177-180 (Aurelius died). This wasn’t uncommon at all and likely has its routes in the Consular system under the republic.

Final thing I think is worth mentioning is the Tetrarchy under Diocletian. Basically he decided to have several co-emperors all at once and… it was just a total disaster. He wanted to have two main emperors, each of whom would have an apprentice who would also be an emperor… it got very messy and this is normally where historians split the empire into east and west because amazingly that actually helps simplify things.

https://www.historyextra.com/period/roman/silbannacus-the-roman-emperor-that-time-forgot/