I recently had an interesting issue crop up for me. I created a SharePoint Designer (SPD) workflow that acts somewhat as a state machine workflow using the "collect data from user" option. This option creates a task, assigns a user, and returns back the ID to the list item created. I was using SharePoint groups as the value for the assigned to field. For some reason the second step was not assigning the user but the other 3 steps were. I checked the length of the name, the users in the groups, the permissions of the group, the owner of the group, pretty much every thing and it did not fix my issue. I more than triple checked my workflow to make sure it was properly named.
Then I realized that there was something different with that group as compared to the other groups. The name had a dash in it. Yep! a dash ( – ). I removed the dash in the name and the SPD workflow was able to then assign the user to the task with no issues. The funny part is it is not a SharePoint thing because I was able to manually assign the user, so the only thing I can think of is that it must be the way SPD assigns the user.
So bottom line is… if you plan on assigning users to tasks from a SPD workflow, make sure you do not have dashes in the name… 
dwollerman