Futurama: 12 Plot-Holes That Almost Ruin The Show
6. Fry's Tattoo
Another time travel plot hole here, this time concerning the actual catalyst for the time travel itself. The time travel is only possible due to the tattoo of Bender on Fry's ass, which contains a timecode hidden in Bender's eye.
There's a minor plot hole that Fry is somehow able to read this code with a mirror while running when the Professor's powerful microscope is needed the first time, but that's not the big one here. Instead, it's where the tattoo comes from at all.
Bender takes it off Lars (really Fry) at the end, but then how did it get on Lars, who was really Fry? And why did nobody ever see it before?
Various strange and overly complicated wormhole theories exist, as does the theory that it was planted there at some point by the cosmic, God like being Bender meets in Godfellas or by the Nibblonians somehow. Both seem rather pie in the sky, though.
Some have tried to explain that taking the tattoo from Lars and putting it on Fry completes the loop, but without an ultimate origin, this doesn't quite wash.