Not sure if this would qualify as a bug or not, but I wanted to report my findings. Without going into great detail, I've been working on installing Omeka on to the BeagleBone Black microcomputer since version 2.0 and today, I noticed this problem for the first time. After getting everything installed and setup, in version 2.3, when I went to add an item with an attached file, over and over, I kept getting the error "There was an error on the form. Please try again." I tried the same procedure on another computer using the same version of Firefox, and ultimately realized that the only difference was the system that got the error was the one that had the "HTTPS Everywhere" add on from the EFF installed in Firefox.
I'm sure this won't happen for everyone, but I thought it might be worth letting you know.
Thanks...