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It appears that having a regular expression starting with ".*" (and presumably other expressions that require backtracking?) causes a lockup when loading certain sites. The common denominator of these sites appears to be their use of TypeKit (a licensed CSS3 font-face solution).

Here's an example rule that causes problems for me:


Even if this is never matched on the page, I get a lockup. The following pattern, which is just as effective, causes no issues:


I've adjusted my blocking rules to work around this issue.