In a situation where you’re charging with a 50 amp extension cord and connector with an adapter to a 30 amp circuit on a 30 amp breaker (30 amp breaker being key) then I wouldn’t worry because worst case you’re tripping the breaker and not causing a fire.It does until the day it doesn't. Plenty of people have been surprised when their car suddently decides to NOT follow the amperage that had been previously set. It's fine for casual use, but to depend on it to keep your house safe day in and out....no way.
On the other hand, 30 amp rated extension cord with 50 amp adapter on a 50 amp breaker is certainly dangerous because you’re relying on software from overloading that extension cord.