Rare misstep for Tay Tay, who's usually on point ... but on Sunday night, she kinda screwed up -- at least in Twitter's eyes, anyway -- when she paid little to no attention to CD who announced her as the winner for AOTY ... which was a big deal for a lot of reasons.

It was the final award for the night -- and the largest prize at the Grammys, period -- which is why it was such a remarkable moment to see Celine come out to present it in person.

Of course, she's very publicly battling Stiff-Person Syndrome -- and has stepped away from the spotlight of late -- so for her to do this was notable and moving, especially since she appeared to look relatively healthy here ... something the audience met with a standing O.

Fast-forward to Taylor being named the winner, when she ran onstage with her collaborators -- plus Lana Del Rey -- and looked so overcome with emotion that she totally downplayed Celine's presence ... simply taking the statuette from her without so much as a nod.

Indeed, it came off as quite rude -- you could tell Celine found it odd as well -- and the Internet unloaded on Taylor for what they perceived as a major lack of respect/reverence.

It would appear Taylor and/or team caught wind of this backlash quickly -- 'cause not too long after ... a photo of the two posing together (and looking cozy) backstage surfaced, which a ton of outlets then posted without much description or captions. Coincidence much?