Many time when we work on drupal site, when we add modules or do any other work we face lots of problem of memory like we see white blank screen, memory error, page not found.
I found this kind of errors when my browser cache is full or i worked continuously on project with opening lots of tabs in browsers. many time i face that error and when i closed some tabs, browsers and clear memory cache, the error is gone.
But lots of time we have to increase the memory limit into our php.ini, .htacess and in settings.php to avoid such errors