When using the check syntax button on filters with extremely large regexes (example: https://en.wikipedia.org/wiki/Special:AbuseFilter/58 - private), it fails with an unknown error.
I checked in my web console (Firefox), it sends the GET request and gets a 400 Bad Request status code back, with the Wikimedia Error screen:
Request: [unknown method] [no URL], from 208.80.154.133 via cp1007.eqiad.wmnet (squid/2.7.STABLE9) to [unknown host] ([unknown])<br/>
Error: ERR_INVALID_REQ, errno [No Error] at Tue, 16 Apr 2013 23:55:47 GMT
Version: unspecified
Severity: major