It said "404 error" in the titlebar, but the page was blank with an extra scrollbar in the page area.
It's gone now, so maybe actually having someone else look for the problem fixed it :)

--Lime2K

So a kind of Heisenbug?

"Mathematics brought rigor to economics. Unfortunately, it also brought mortis."
- Kenneth Boulding

I was unaware there was a name for this phenomenon! Thank you very much! This is something I've had to explain for years.

(Actual appreciation here, not sarcasm -- I know it's hard to pick up in text)

--Lime2K

That sounds like something's trying to wrap the page in an invisible iframe.
I'd suggest both scanning your machine for malware, and making sure you're not behind a (hotel-style) web proxy that's trying to inject external content into your web browsing experience.