Perhaps "accidental" is not the word so much as "involuntarily". And there are degrees. The FF were already a team, but got their powers involuntarily. The Avengers already had powers, but only begrudgingly became a team. Everything about the X-Men is deliberate except for the mutant power lottery--Xavier actively recruits team members and they are free to come and go as they wish. The Suicide Squad was forced together by outside influences, literally at gunpoint.
Probably the most "accidental" team would be the GotG, a bunch of misfits that started out stealing from and shooting at each other, who only initially formed a "team" out of expedience.