There's been some buzz today about the new version of Google Toolbar (beta5), and how it takes over, some might say hijacks(!) 404 error pages. I was concerned at first, but fortunately Matt Cutts has written a good post fully explaining the behaviour:
How 404 pages work in Google Toolbar Beta 5
The basic crux of the matter is - make your 404 error pages greater than 512bytes in size - something any good webmaster should be doing anyway! So a bit of a false alarm, but good to know.