Bucklers were often used by unarmored or partially armored medieval combatants. Were they ever used by fully armored fighters? It seems that in battle the answer was sometimes yes.
Some of the iconographic evidence is suspect. Bucklers born by St. Michael or Goliath or ancient Romans may owe more to the illustrated text than to normal medieval practice. However, even eliminating these suspect cases still leaves a number of medieval illuminations of medieval battles. Here is a depiction of the battle of Otterburn.
Here and here and are other incidents from the Hundred Years War. Here is the capture of Charles of Blois in 1347. Here is the elite guard of mounted Scots archers employed by Charles VII cast in an adoration of the Magi by Jean Fouquet. They are armored from head to foot, and at least one wears a buckler. It looks like fully armored men did sometimes equip themselves with bucklers, although the practice seems to have been rare.
Were they ever used by fully armored combatants in tournaments or other consensual deeds of arms? Probably not; I’ve read scores of accounts of such deeds of arms from the 14th to the 16th century, and none of them mention bucklers.
There is one very suspect piece of evidence in favor: One illustration from Maximilian’s Freydal shows him triumphing over an opponent, with both fully armored and armed with sword and buckler. However, the work is not a factual narrative but an allegorical exaltation of Maximilian’s omnicompetence with every conceivable weapons form.