I'm not the only one to discover this bug (or easter egg), but if you go to the
level editor, select the character and manually type "5" in the input field to the right of "default character:", you will see the motorcycle from
Akira! Although you can't test your level or play using this vehicle, it suggests this is a possible upcoming vehicle. When asked to comment on this, Jim Bonacci, creator of Happy Wheels, had this to say:
"EAT F***ING S***"
Seriously.
[UPDATE: With the release of the Moped Couple, this trick no longer works]