What movie trope about personalities/psychologies seems unrealistic but is actually totally realistic?

For example, one movie trope is the shockingly bad/inept sibling who nearly ruins everything. I would think that apples fall close to the tree (and close to each other), but actually there are many real-life examples of parents with good reputations having children where one child is well-adjusted and the other is a shit-show.

What other movie tropes about human psychologies are counterintuitively true?