Humorous related story -- I was a customer of GM's OnStar service, and had a login on their web portal that I used for account maintenance and to check on vehicle stuff. Sometime later, the GM team rolled out a new version that blocked periods, but my account name had a period. It would happily send me an account recovery email with my now invalid username that I couldn't use. Nor could I configure a new account as it would happily point me to my old, now invalid account. The version without the period did not work.

This was a pretty fringe service of negligible value to me, so when my CC changed I sent them an email telling them of this problem, and my inability to change my account. No response.

So I let the account expire. Still couldn't believe that a team could make a new presentation that blocked already in place usernames, nor would they do anything about it.

They probably recognized the + and stripped it, as well as all subsequent characters, from your address upon form entry.

Same here, it's quite often, actually. Enough that I won't do it anymore.