We've replicated an error encountered by a Safari-using donor from Zendesk #357265: if you type currency code USD in addition to the amount on the Other field, you get a minimum amount error message:
The banner is https://en.m.wikipedia.org/wiki/Apple?banner=B1718_0816_en6C_mob_p1_lg_txt_cnt&force=1&country=US
Re-opening, that change broke using backspace and arrow keys in Firefox so I've reverted it while I look for another fix.
Wow, for something so basic KeyboardEvents are a total mess of supposedly deprecated/unsupported properties. This change should work though, it explicitly excludes Backspace/Delete/cursor key events. Tested in Chrome, Firefox, IE, Safari.