Not sure we can call this a blooper because it was intentional, but in Out on a Limb, the mailman picks up the mail from a set of boxes that are attached to the wall just to the right of Furley's apartment door. Since when has there been mailboxes there? I'm not sure if they kept it there for the rest of season 8, but I know it wasn't there when the Ropers lived down there.

One other scene I remember offhand where they were looking at the mail outside is in "It's Only Money", but the mailbox isn't shown directly. Nonetheless, Janet looks at the letters right outside the apartment door.

Oh, and in "And Mother Makes Four", Janet is punching out Jack's name for the mailbox, but again the mail box is not shown.

The thing is, though, the boxes might have been moved at some time in the intervening six and a half years, so I'm not sure that this counts as a continuity error. (For example, 201 itself shows traces of having been renovated more than once.)