I used SharePoint Designer 2007 to create a custom workflow for a list on one of our sites. The workflow is very straight-forward: it executes automatically when an item is created, and is set to send an email if a particular drop-down is set to "yes". When an item is added to the list with the field set to yes, no email is sent. Looking at the logs, an exception is being thrown with the following message: "The requested workflow task content type was not found on the SPWeb."
The odd thing is that we have workflows that are almost identical setup on lists on a different site (same site collection) that work flawlessly. I have looked at the setups for both sites and as far as I can tell they are setup identical.