...so ISP's should filter AS paths!
Filtering the path would not have prevented this from happening. However, filtering paths whos length was unrealistically long would have done a world of good.
I always thought they did.
Most already do. The problem was not the ASPATH itself, it was the length of it. The routers affected did not handle updates for a prefix which required more than one AS_SEQUENCE segments in order to obtain the full AS path. The existence of the additional AS_SEQUENCE segment is what triggered the bug, causing the receiving router to treat the update as invalid, and the BGP session is dropped.
You must realize that the computer has it in for you. The irrefutable proof of this is that the computer always does what you tell it to do.