I tried but couldn't find a file that has line 307. They are all very small files.
In Version 2 (beta 10) they aren't. prototype.js has 2515 lines!
You are trying to confuse us. Please always add the forum's version, you are talking about, or even better post your questions concerning v1.7 in the old forum only.
var src = Event.element(event);
this.pattern = pattern || Template.Pattern;
The only candidates for an error are effect.js and prototype.js. Since nobody complained until now, I suspect they are working (I would not start fidling around with them).
Probably something in calling them is not working (any more). Did you change something in your templates?
Alfie (Helmut Schütz)