My favorite story of finding an arena bug wasn't actually finding an arena bug, but was instead Arena finding a bug.
When they first implemented [[Conqueror's Galleon]] from Ixalan, every time it would attack, after combat... it died. It just died. Went to the graveyard.
Conqueror's Galleon back then read, "When Conqueror's Galleon attacks, transform it at end of combat." This meant that it was still the same permanent. Meaning that it was still under all of the same effects. Meaning that it had been crewed. Meaning that it was a creature. And since Conqueror's Foothold doesn't have power and toughness, its toughness defaulted to 0 and it died instantly.
This was found before print, which is why Galleon exiles itself to transform.