Update: Claims of the image being AI-doctored appear to have been wide of the mark: this may just be a good old-fashioned optical illusion. The headline and [[link]] some elements of the copy have been changed to more accurately reflect this.
Assuming Marvel isn't teasing a third act twist where this one particular Thunderbolt sprouts a new digit, the poster shared by the official Marvel and accounts today appears altered from its intended appearance. Interestingly, that version is the only version with the error. A more narrow version of the poster that cuts [[link]] off before the apparent sixth finger appeared in alongside the new teaser.
As weird as the bonus finger is, it does seem to be some sort of unintended optical illusion, possibly down to some re-sizing around the digits. A possible explanation is that someone's done a dodgy Photoshop while resizing the standard poster for other formats, but it seems unlikely this is the work of generative AI.
The posters with the possible phantom finger remain online at the time of writing with no comment from Marvel as of yet. You can watch the teaser trailer for Thunderbolts* below (wow, that asterisk is actually part of the title).
