Over the past few months occasionally I'll discover a random jpg's contents have been overwritten to contain, in plain text, "An error has occurred. (403)" (28 byte file size). The first time I encountered it was while going over a backup copy of a drive that had died, and assumed some errors had occurred when I copied it (the source drive was dying, and I ended up having to use an Ubuntu Live CD instead of Windows XP to get anything copied at all). However, now I'm seeing random overwritten jpgs slowly popping up on my newer drive, on Windows 7. Google doesn't return any programs that I use when searching for the plain text (mostly hits on http 403's and a backup program, such a generic error message...), I myself haven't touched most of the jpgs recently except for viewing, and the timestamps (created/modified/accessed/whatever) haven't been changed whenever I find another problem jpg.
I have no idea when the file gets overwritten, I only notice new ones when going through a directory with an image viewer (almost always IrfanView), though now I'm keeping an eye open for that reported file size and having TeraCopy test after operations. The number of jpg's with issues is still in the lower double digits, but I can't have jpgs randomly being overwritten (and possibly other file types, but jpgs far outnumber most other frequently accessed file types on this computer due to work and are encountered the most).
I'm stumped, as searching hasn't returned anything useful, and the programs I think most likely to have accessed the files have never caused a problem like this before (windows file operations, TeraCopy [installed after problems], Comodo, IrfanView, Defraggler) nor have related searches shown similar problems being reported.