It's right above, and is the "List-End" Instant UBB Code button. Certain browsers (mostly IE) extend that button's hot zone down to just above the Add Reply button. If you do not squarely click Add Reply when you post, you may get a ListEnd tag inserted at no extra charge.
This appears to be a browser bug. I use iCab and have never yet posted a[/list]unintentionaly. Or perhaps I just never missed the Add Reply button.