Deliberate mistake: In the scene where Emmet drives to work, you get Emmet's view of the dashboard with his hand switching on the radio. The car at that moment has no front window, perhaps to make the car look bigger from the inside. (00:04:50)
Deliberate mistake: While done for comedic effect, the ram's penis as we see it is not what a real one looks like, most noticeably is the missing urethral process.
Deliberate mistake: When Jim and Lauren are having their stargazing experience, it's clear that the pints of beer the monkey brings them are fake so that they don't get spilled.