Well, the SoD (Stronghold of the Damned) is what it is...
Spending a night with an empty hero for the Warlock isn't a problem either – you get a legitimate 1300 mana points and can run around... it's better to leave at least one unit with a speed of 4, and that's already better, and I won't even mention the possibility of losing such a hero in an attack, even without a chance in battle...
The ability to break down walls with Cyclops in tactical mode (Cyclops can shoot at walls in the tactical phase of the battle, destroying the entire city; this is an oversight by the developers of Heroes 3) is an obvious fact against a human player, but against the AI, everyone doesn't care, especially when you already have Cyclops, the AI usually doesn't pose a problem.
There are also bugs: using navigation mana points on land (whether it's manipulating the Admiral's Hat or escaping from the sea).
Taking good items from an ally (if the game is with allies, but this is rare).
Raising additional undead from summoned creatures in battle (elementals, clones, demons). To do this, you need an enemy Necromancer hero and you need to drain his "summoners" (usually with area-of-effect magic). He will get undead, and after escaping from him and attacking a second time, you can use this extra undead for experience/your own undead.
The "dismiss and re-hire" trick can be used (although it's very unlikely in online games) to have a more developed hero when dismissing/killing him; then, when you re-hire him, he will have 2000 mana points