After asking this question, someone pointed on the right direction of not being able to execute a second script at all if one was already running.
I usually make apps which rely on the execution of AJAX calls to PHP pages, and today I found that trying to write on a file with
echo "hello"; exit;
As Luis said it could be a write-lock on the file that you're trying to modify. However another possibility if you're using sessions that use files (rather than a database), or a framework that uses file-based-sessions - then this behavior could also be a result of session-locking. My money would be on Luis' answer though - you should probably be using a database rather than a file unless you have a solid reason not to.