I saw somewhere recently that the "plague of frogs" was likely someone found a single frog that was larger than normal and claimed that was the sign.

But over the centuries and through all the translations, it became an unbelievable amount of frogs everywhere.

I just picture one crazy guy with a pretty normal frog running around screaming "this is a sign!!!" And everyone ignoring him, then 2,000+ years later everybody just accepted it.