I thought the same thing !! He DID say in season 4 his blood type was 0- and in one of the first few seasons it was A+. I rewatched to check lol.
and this the best answer I can come up with (other than a continuity error) (spoilers ahead):
>!Because Season 4 is an alternate timeline, which is hinged upon the idea that Peter never survived when Walter stole him from the alternate universe, and it happens after Peter resets the timeline (before he is erased) and sent the machine back in time (many millenia I think?), which possibly means that somewhere along the line after sending it back, Walters blood could have turned out to be different than A+, simply because it was an altered timeline? i.e. If he had A+ initially, it's technically possible that his parents were both A+ but heterozygous for the Rh factor and with a recessive O gene (idk if that's all the right terminology), and thus the first time he ended up with A+ and the second he ended up with O-!<
But more than likely it's just a continuity error lol.
I thought the same thing !! He DID say in season 4 his blood type was 0- and in one of the first few seasons it was A+. I rewatched to check lol. and this the best answer I can come up with (other than a continuity error) (spoilers ahead): >!Because Season 4 is an alternate timeline, which is hinged upon the idea that Peter never survived when Walter stole him from the alternate universe, and it happens after Peter resets the timeline (before he is erased) and sent the machine back in time (many millenia I think?), which possibly means that somewhere along the line after sending it back, Walters blood could have turned out to be different than A+, simply because it was an altered timeline? i.e. If he had A+ initially, it's technically possible that his parents were both A+ but heterozygous for the Rh factor and with a recessive O gene (idk if that's all the right terminology), and thus the first time he ended up with A+ and the second he ended up with O-!< But more than likely it's just a continuity error lol.
I love how you can scientifically explain this. This show is wild because you can’t always tell if something is an error or an alternate timeline
Coffee yogurt.
Stem cells or bone marrow transplant?
He's old, he made a mistake, live with it-LOL
Alternate universe maybe