These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
The Computer Is a Cheating Bastard: The enemies are only supposed to attack barricades if there is no other path to get to the rift. On occasion, the game will bug and enemies will destroy barricades despite there being a clear path for them to go around. This is likely a minor feature that triggers in the event of a pathfinding A.I. Breaker.
Demonic Spiders: It's in the job description for Gnoll Hunters, who directly attack you instead of going for the rift.
If you're going for a five-skull ranking (which requires that you let NOTHING through the rift), Kobolds can quickly become this. They hopelessly outrun most traps and Paladins. You'd better hope the archers stop them if you're not around to do it yourself, or spend a ton of cash laying down Grinders and Brimstone (and then keep an eye out anyway to make sure the little buggers don't overwhelm them with sheer numbers).
In the sequel, post Family TiesBooster Pack, Hobgoblin Healers become this whenever they are paired in a wave with a large damage sponge creature, such as an ogre (or worse.) Desperately burning down its huge reserve of Hit Points, getting close to finishing it off, only to have it restored back to full health by a couple of Hobgoblin Healers you never noticed approaching from behind who are hanging out on the safe end of your Death Course...
Funny Moments: The Apprentice dancing when he completes a level. Never mind the gravity of the situation - he's loving every moment taking his enemies down.
Goddamned Bats: The Fire Babies. They are slow, do little damage, and die in one hit, but they fly, they are small and hard to target, and they come in big numbers. They are not much a threat on their own, but their flight means they have to be dealt with sooner rather than later, distracting the player from other threats they might be focusing on. Fortunately, the Lightening Ring takes them down efficiently.
The sequel adds bats of different elemental types, immune to their own element. The lightening ones are especially so as they chain damage to ground targets, automatically hit, and are immune to lightening in return. However, the sequel also adds additional options for taking down fliers, so this is not quite as bad as it sounds.
Good Bad unintentionalArtificial Stupidity: Gnoll Grenadiers can't tell which floor their target is on, which can lead to them throwing bombs into thin air one floor below your archers/what have you. Perfect if you have other dangerous enemies to take care of who won't be a threat to your guardians.