krumo()/dpm() not working I have a custom module and a template to alter the appearance of my node submission forms, á la these instructions.

My module consists of three functions: 


A hook_form_alter() that works fine
A hook_theme() that doesn't do anything but return an array, even if you enter other code before return (not sure if this is by design)
A hook_preprocess_HOOK() that is currently empty


dpm() doesn't seem to do anything in hook_preprocess_HOOK(), although krumo() on the same variables sort of works. It sets a Drupal message that reads Array: [n] items but can't be expanded or inspected at all. 

In my template, print_r($form); prints out the form array as expected. dpm('self-aware roomba'); sets a Drupal message of "self-aware roomba" as expected. but dpm($form); does nothing and throws no error.

Everything except my hook_form_alter() is exactly as it appears in the linked tutorial. I even tried pulling out the entire hook_form_alter() to see if it works without it; it doesn't.

What could be causing dpm()/krumo() to fail silently?
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.