I have on a couple of occasions encountered someone out of my life still harbouring a grudge (usually work-related).

They get extra mad when they realise I have already forgotten them and barely remember them even when reminded.

What can I say? Rent in my headspace is very exclusive, the world has too many _actually_ _interesting_ things... 

 It is very common among colleagues or former colleagues, but it is widespread across all areas of life Low Information Voter.