A big budget doesn’t always guarantee a perfect movie — especially when special effects are involved.

Some of the biggest sci-fi movies, including this weekend’s thriller "Alien: Covenant," can’t escape the occasional plot hole, continuity error, and, in some cases, blatant gaffe.

For all you purists out there, here’s a roundup of some of the sci-fi genre’s most noticeable movie mistakes. All images courtesy of MovieMistakes.com.