1) Weather may suddenly change
2) Clothes may suddenly change so for example colour of clothes
3) Props in scene may suddenly change, so for example may be in another position all of a sudden
4) Facts at start of film may change later on in films. So for example codes, passwords etc.
When the audience watch films they don't notice any continuity mistakes that may have been made in the film. The meaning of continuity is that the films continues with no confusion or mistakes and so that it makes sense and makes it more realistic to the audience.
Above I have described some examples of things that can go wrong in the film which the audience don't even notice because they're so focused as to what is happening in the film, they don't notice the continuity mistakes.
Here is a video showing some examples of continuity mistakes from the Star Trek franchise that the audience have not even noticed.
And here are some other continuity errors from other famous feature films:
