An amusing editorial in the neuroscience journal Cortex discusses the excuses scientists use to explain why they didn't submit their peer reviews on time:
Following our nagging for late reviews, we learned that one reviewer had to take their cat to the vet, another was busy buying Christmas presents, one was planning their holidays, an unfortunate one had their office broken into [...] others agreed to review whereas indeed they really intended to withdraw, or were just too busy to reply.
The piece is by Sergio Della Sala, Cortex editor in chief. He goes on to describe one case in which a slow peer reviewer led to an especially frustrated author:
Recently, we received a fuming message from an author protesting that they did not get feedback about their manuscript within six weeks from submission. Messages from this author, rightly displeased by our slowness, became frequent, wordier and more irate ...
