I am not saying this is a common thing, but it's possible that you will find irregularities like the ones in these pictures; pieces connected to each other but not following a straight line.
These pieces are harder to connect because you tend to think that they don't fit. But then you try and it works! So my humble suggestion is not to rule out any possibility and rather pay attention to the colours or the patterns rather than to the shape of pieces.
Honestly I don't dislike these irregularities because they slightly raise the difficulty level of a puzzle.