Damaged file?

I received a file in epub format, and brought it into calibre no problem. I converted it to mobi, no problem. I opened it in calibre, and it opened just fine. But when I put it on my paperwhite, it won't open, and I get a note saying it's a damaged file. Same thing happens when I put it on my K3. It opens just fine on my Sony 350.

Any suggestions why it's misbehaving?

eta: I deleted the mobi files, converted to htmlz, deleted the epub file, converted back to mobi and epub, and now it's working just fine, so there must have been some minor glitch in the original conversion. Whatever, it's working, which is all I care about.