krumo()/dpm() not working
I have run into an issue where dpm() and some other messages were eaten up by a 404 request in the background.

Explanation:

If dpm() or drupal_set_message() is called before the messages are printed out with theme_status_messages(), then you get to see them on the same page.

If dpm() or drupal_set_message() is called after theme_status_messages(), then those messages are delayed in $_SESSION until the next request that does theme_status_messages().

Some types of requests do NOT trigger theme_status_messages(). E.g. a form submit will only do the form processing, and then do a redirect, so the messages remain in the $_SESSION.

Also, it will only trigger on requests by the same visitor/client (this is why it is saved in session, which is client specific).

However, some requests that happen in the background do trigger theme_status_messages(), and can eat up your messages.

In my case this were requests to missing images, which resulted in full-blown html 404 pages WITH messages (and I did not get to see any of this, obviously).

Solution:

The solution was to activate the "fast 404" feature.


Change notice 1296384 explaining this newly introduced feature.
Drupal 7 default.settings.php you might want to copy some code from here.