Revealing mistake: When the movie starts there's a car chase where a baddie throws a grenade at a car. When the burning car flips and rolls it is very obviously hollow and the driver has been replaced with some structure that doesn't resemble a man at all.
Revealing mistake: In Darkman's daydream where he launches himself across the boardroom table and chokes out Durant, look at his mouth when Durant rips his mask off. You can see Arnold Vosloo's real teeth behind the fake ones.
Revealing mistake: After stealing the battery out of Durant's weapon near the end, as Westlake says "Batteries not included", you can see the actor's undamaged lips under the fake burn prosthetics on his face and mouth. (Look underneath the exposed teeth where his lips supposedly were burned off.).
Revealing mistake: Near the beginning, when Westlake is experimenting and his mask melts, you can clearly see that the mask is not really melting, and is just covered with suds while a "melting" sound effect plays.