Error is presented when trying to change trigger's prototype name:
1. Try to change name of some trigger prototype (eg in Template OS Windows). Just change single char.
2. Press Update
You would get an error:
Now please note: How you can update the trigger from this point:
A. If you change name BACK (exactly as it was) you could save the trigger proto
B. If you leave the name changed but would go and change trigger proto EXPRESSION as well - you would be able save the trigger proto