Mulder stayed at the house of a police officer in Vermont (or some other northeastern State.) The neighbor was flying a Canadian flag from their front porch. It's because this show is filmed in Canada, so this should qualify as a goof.
Haven't seen the episode but the flag isn't flying because the series is made in Canada because the last season and the current season are being made in LA not Canada anymore.
Hey, I'm a dual citizen Canadian-American with New Zealand, Germany, Russia, England and a few others tossed in for flavor... What flag should I fly, the United Nations? I live in Canada and I see flags from practically every nation when I drive around town (Calgary).
I live in Vermont and people in my neighborhood have a very wide assortment of flags. The lady next door has a british flag on her porch. We have the usa. It is a matter of opinion. People do what they want.
While all of the above explanations are perfectly valid, let's not forget we're talking about TV here. I've never seen a TV show that treats it's audience as though they have enough brains to figure out something like that.
It's definitely a slipup.
Well, if you take it that far, you could also say that no matter where the eps made in California take place, they all look suspiciously like Malibu Creek State Park.
Actually I think it's more of an easter egg. The X-files were filmed in Vancouver (where I live and actually saw a couple of filmings!) for 6 seasons, and Chris Carter really loved it. I think it could be his homage to the country he filmed in for six years. But then again I could be wrong.
This was not a goof up. The series being filmed in Canada had nothing to do with why the flag was up there. Maybe the people he was staying with were Canadian.
Was this 6th,7th, or 8th season? If it was then your "slip-up" is merely an inside joke because only the 1st through 5th seasons were taped in Canada. And also, New England is pretty close to Canada, perhaps he is Canadian and moved to the U.S.