I've heard of this "Could not open 'somefile.jpg' because Brendan's an idiot" message before, so it's not your imagination. I'll assume the person who wrote that error message text into Photoshop was Brendan himself, unless he was blaming someone else for the problem...

I tried it, using Super Get Info to change the Type code on an old 1989 PICT file I've kept around (an old floppy disk startup screen!) to PNGf, but when I tried to open it with Photoshop 7, it didn't give me the error message blaming Brendan--instead, Photoshop displayed "the file-format module cannot parse the file". Then I used Photoshop to create two files, one a PICT and the other a JPEG, and then changed their Type to PNGf and tried to open them, with the same results. I thought of using some utility to search through Photoshop 7 for the reference to Brendan, but then thought better of it since my time was better spent simply believing you're right.

Oooohhhhhhhhh! Duh. I had totally forgotten that I had installed SuperPNG. There it is sitting in my plugins, with Adobe's PNG disabled. I never save directly to PNG any more, I always save for web.

Disabled SuperPNG and enabled Adobe's and it's just the standard parse error now :(