@gollum,
Your question is ambiguous. If you mean have there been bodies of troops within a beseiged city who came to the aid of an attacking force, then certainly--two examples which come immediately to mind are the battle of Lincoln in 1141 during the Anarchy (supporters of Maud the Empress held the citadel in the city, and poured out into the city when Stephen of Blois' army was defeated just outside the city); and the siege of Soissons in 1414--someone took the Paris gate in a rush and opened it to French troops--the drawbridge of the citadel was down and the portcullis was up and the French were able to take the citadel in a
coup de main assault. (The ugly irony is that the Duke of Bourbon then gave the city up to the sack, although the inhabitants were widely known to be loyal to the King. The French troops pillaged, raped and murdered for three nights and two days.)
So in a strict and inexact sense, the answer is yes. Whether or not there was actually ever a fifth column of trained, equipped troops secreted in a city, which is what General Mola wanted people to believe when he spread his propaganda, however, is a different matter. I can't think of any such event.