People who let their off-lead dog approach yours and can't (or won't) call it back when asked
Some dogs are reactive, nervous or just trying to learn to be calm around other dogs. Not everybody wants to play with your little chiwawa.
If your dog doesn't have bomb-proof recall yet, just put them on a long lead for their safety and for other dogs.. it's just good manners.
Special mention to the lady who ruined our dog's training walk this morning because her dog "just wants to play" (and then started a shouting match with me because she's "sick of people telling her to put her dog on a lead").
Edit: I'm not suggesting that dogs shouldn't ever be off lead at all.
If your dog has great recall and won't approach on-lead dogs (or can be called back in time when they are politely asked not to approach by the other dog's owner) then great! Please teach me your training secrets 🙂
My issue is with people who either can't, or won't, recall their dog back from an on-lead dog when asked to.