Signese

The typo is 愿. Should be 怨.

That's what I thought - though I would have expected to see 怪 there, not 怨。

though I would have expected to see 怪 there, not 怨

Aren't 愿 and 怨 homophones, thus the typo? That's what I thought.

Yep, but I've never heard 别怨我不客气, only 别怪我不客气, so I wasn't sure if I was missing something somehow.

But this is handwritten, right? Is using a homophone a common error in handwritten Chinese? I always assumed it would be more common in a phonetic (e.g. pinyin) based computer IME, but don't really have any evidence to back me up.

Not sure if you'd call it common, but it certainly happens - you don't, for example, know which character the yuàn you use in speech is. See also the 务/勿 mix up some time back.

could be 別(埋)怨我不客氣.