I've never seen a misaligned slider on an HHKB, that's really hard to do with this board, but I've seen a warped keycap, so that's my vote. The best way to verify this is to switch this keycap with another one of the same row and size.
I don't know of any way to just get one keycap from PFU however, so if you don't have a warranty, and it bugs you, then you're probably going to have to spring for a complete keycap set.
Edit: BTW, that left Shift key can be swapped with Enter for a comparison.