[edit] looks like this has been changed / fixed.

this isn't really a bug so much as it's just... how things work, but i don't think it makes sense for it to work this way.

basically if a thread gets locked, but you still have it open in a window from before it was locked and the reply box is still there, you can still add your post to the thread. then the page will load with your comment added and the thread showing as locked.

(this happens all the time to me with those holiday "[X] the user above" threads bc i tend to keep them open in another window and periodically claim on them, but i don't always remember to check how many pages have been added since my last post, so sometimes i'll make my post and then realize the thread is locked, and i'm the last poster.)