I enabled notices by email for PM's, in my profile's Personal Message Options, and suddenly the user blank123 has been added to my ignore list.
When I try to remove him (her?), he keeps getting added back, unless I put a different name in his place.
I don't particularly mind ignoring blank123; I don't know him, and he's been inactive for more than five years, but still... it's an odd problem, and I'd rather be able to empty the list.