The “Photo not uploading” thread had a spam comment (rapidly deleted), but this has somehow interacted with the way Discourse highlights new/unread posts, and its thread title is stuck on black, rather than grey. (I’ve scrolled down the thread list, and there are no more instances - for me, other people might see things differently - so there may be some timing issues involved in this.
I guess this is something just to report onwards to Discourse.
Clarification: this shows up on the Latest display - New and Unread give empty lists - so it’s just an oddity in the Latest display.