Originally Posted by Slake

This I can answer; SpellWatch determines when you've cast a spell successfully by using two events; first it captures your combo points / target / spell being cast when the UNIT_SPELLCAST_SENT event is fired; this is when you send the cast command to the server. However, it doesn't start a timer until it gets the UNIT_SPELLCAST_SUCCESSFUL event; which occurs once you finish casting. The problem, I think, is that when you use /stopcasting, your client never issues the UNIT_SPELLCAST_SUCCESSFUL because as far as your client is concerned you can't have finished casting the spell because you told it to cancel. This works in practice however because of client-server latency.