Missing skill. Id=1259; Name=gw

Every now and then when I post a reply to a thread, instead of it going through normally, I get a black white page with some text along the lines of "Missing skill. Id=1259; Name=gw" and then can't access it anymore. It brings me back to the same white page with the text every time I try the thread again.

What does that mean? Am I the only one getting this? Can anyone else access the thread when that happens? If not, does that mean I killed the thread? If I killed the thread, do I get in trouble for it?

It means someone put something in brackets - ] and [ - and the forum thought it was a nonexistent skill icon. If you're doing it on purpose you get banned, otherwise we'll just fix it after a while.

Is there any way I can access the post and edit it to fix it?

No - it has to be deleted by a moderator/admin.

But you can report that thread, to draw moderator attention to it, although there are plenty of mods around, so it shouldn't be necessary.