This user experienced an obvious FF issue with his DA 21 Limited lens and conducted a series of simple but yet effective test to confirm the problem:-
http://forums.dpreview.com/forums/read.asp?forum=1036&message=20984715
In contrast, another two different users experienced BF issue with their each own copy of the DA 21 Limited lens:-
http://forums.dpreview.com/forums/read.asp?forum=1036&message=20912492
But the common point of all the three reports is that the users are all believing that the problem is mainly associated with the DA21 lens model.
So, does the DA21 Limited itself have some inherited problem by itself? Or, simply all those are just poorly calibrated units from the factory?
Nevertheless, one thing is of no doubt, for a nearly $500 priced prime, if I was the purchaser or even just the user, this would be a big joke to me.