My bet would be on the regexp that matches valid SVGs
Nov 15 06:38:23 ms-fe1001 proxy-server: HTTP status code mismatch. Mediawiki: 200 Thumbor: 500 URL: http://upload.wikimedia.org/wikipedia/commons/thumb/2/20/Lori_in_Armenia.svg/107px-Lori_in_Armenia.svg.png Nov 15 07:15:50 ms-fe1001 proxy-server: HTTP status code mismatch. Mediawiki: 200 Thumbor: 500 URL: http://upload.wikimedia.org/wikipedia/commons/thumb/c/c9/Texas_in_United_States_%28US48%29.svg/125px-Texas_in_United_States_%28US48%29.svg.png Nov 15 08:12:59 ms-fe1001 proxy-server: HTTP status code mismatch. Mediawiki: 200 Thumbor: 500 URL: http://upload.wikimedia.org/wikipedia/commons/thumb/9/99/Question_book-new.svg/789px-Question_book-new.svg.png Nov 15 09:42:09 ms-fe1001 proxy-server: HTTP status code mismatch. Mediawiki: 200 Thumbor: 500 URL: http://upload.wikimedia.org/wikipedia/commons/thumb/9/94/Jabrayil_in_Azerbaijan_%28semi-secession%29.svg/535px-Jabrayil_in_Azerbaijan_%28semi-secession%29.svg.png Nov 15 10:47:42 ms-fe1001 proxy-server: HTTP status code mismatch. Mediawiki: 200 Thumbor: 500 URL: http://upload.wikimedia.org/wikipedia/commons/thumb/1/13/World_Organization_of_the_Scout_Movement_map.svg/96px-World_Organization_of_the_Scout_Movement_map.svg.png Nov 15 12:09:16 ms-fe1001 proxy-server: HTTP status code mismatch. Mediawiki: 200 Thumbor: 500 URL: http://upload.wikimedia.org/wikipedia/commons/thumb/e/e9/Northumberland_in_England.svg/62px-Northumberland_in_England.svg.png