Extra Hero Movement Bug:

After moving troops and heroes around within an army, the hero may regain their full movement potential under certain conditions.  This happens even after they have moved for the day.  The effect of this bug gives more movement then the hero is supposed to have for a day.  Players using this bug in a ToH game are considered to be cheating, and will be penalized by the Honor Council for such behavior

Oracle 'Missing X' Bug: 

When using the oracle to identify the marker 'X' to dig up the artifact, there is approximately a 10% chance that the X will not show up on the map.  A minor bug, but be aware of it nonetheless.  Sometimes the X will be under a tree or inaccessible area, therefore cannot be dug out.

Kingdom Overview Troop Transfer Bug (corrected with 2.2 patch): 

Through the kingdom overview, a bug lets you transfer troops & heroes between heroes and garrisons.  Hence it allows for transfer of creatures or a hero to another town or hero instantly.  Major cheating bug all players should be aware of.

Autocombat Crash Bug (corrected with 2.2 patch): 

When two humans face off in combat. If anyone hits the 'auto combat' button, the game will lock up.

Stealth Skill Bug (corrected with 2.2 patch):

If you use stealth with your hero to avoid and gain experience from the monsters that show up because of 'month of the X', it will cause the 'waiting for remote player' message to appear on your opponents screen (which never goes away).

As a result, when your thief gets to within 500 xp from his next level, you have to be extremely careful where you walk him - looking for an unguarded treasure or xp powerup. It's only through stealth that it crashes, and it's any monster. Last game, he was able to do it provided that the skills offered in the new level were not the next level of stealth. Example, move up a level and get offerend Combat, Seamanship, or Pathfinding - but not stealth.