Updated a Gmail filter that automatically marks certain incoming mails for follow up, and it marked 500 threads, diluting the 25 I had marked.
Result: * Wasted 1.5 hours sorting again all the 500 e-mail threads (to restore the 25 that really required follow up).
Learnings: * Filters should not apply labels that you would also apply manually as a part of your document workflow. * Learned new syntax for searching e-mails, you can add date restrictions, e.g.: before:2014/5/17