We've been following up with about a dozen donors over the past two weeks, trying to isolate what's causing these errors:
So far they seem to be using up-to-date versions of Chrome, and responding to banners rather than email links. Could it be related to the KeyboardEvents mentioned in T173431: Safari issue with Other amount option in banner: mask currency code letters?
If there are more specific details we can ask for, please let me know. So far no one's mentioned anything like using the Back button, the error seems to occur on first go.