Reported by @glossy sedge
I don't know exactly how to reproduce it, but I could consistently get this to happen in conversations with multiple file attachments. (Edit: I also noticed that in the conversations I had with this bug, it also happened after 10 messages regardless of whether the files were images or text files, I still don't know if the file attachments have anything to do, but it at least makes this bug easier to notice)
The AI to actually keep track of what I just sent it. If it had something to do with the context limit, it should forget the earliest stuff, not the latest.
At one point, it suddenly changes its writing style much earlier than usual in the conversation, which normally only happens after reaching the context limit. When I ask it if it remembered what I just showed it, it got everything wrong. This happens regardless of custom instructions or model selected (both GPT-5 and GPT-4o gave the same results). However, if GPT-5 decides to use reasoning, it can remember the earlier context of the conversation, so maybe this only affects non-reasoning models.
Tested in the ChatGPT Windows app and website versions.