Just spent a few minutes trying to troubleshoot a fiddly problem with SharePoint Designer, the “I have a Hammer and Everything’s a Nail” tool of choice for SharePoint 2010.
When you are comparing 2 “People Picker” fields in the current item, you do not get to choose the format for the comparison from the first field – but the field you compare it to (in the same or another list) has 5 formats available to choose from. This issue does not appear when you are comparing “any field in any list” because you get to choose the data format on both sides of the comparison.
The Comparison is done using the “Login Name” data format… which brings me to my next issue. A user had recently had her profile recreated in AD. Normally this is fine, however when they recreated it they put a capital letter in her login name – and because the comparison is case sensitive, it was not picking up the fields as a match (this was a tough one to find).
Essentially the workaround for that was to create an initial “step” that did a case-insensitive comparison and then “Did Nothing” (because the only case-insensitive comparison option in SPD is one that equates to True when the fields are equal… and I wanted the workflow to fire when they were not equal…).
I guess after spending so much time doing SQL queries in SQL Server (which are case insensitive by default) it just did not occur to me that this might be the problem. Ah well – something to watch out for next time.