Enemies sometimes get stuck behind some areas.
Poor spelling and grammar in the subtitles.
With the 'serious time' device active, enemies that have been killed move in real-time despite a slowdown being in effect.
A windmill can be seen turning even though it has no sail.
The tank has Serious Sam's logo on it, even though no-one could have known that he would be there.
Shotguns do not have tracer ammunition.
The person in the tower was flashing 'S-O-S-O'. It should have been 'S-O-S S-O-S'.