Hello,
I use ./bash 4.3.42 with bundled libreadline and git/contrib/completion/git-completion.bash , as modified by commit 8716bdca268 from 22 Feb 2016.
I cd to git.git (non-bare, with working tree) and type
git b62c.. ori<TAB> to force TAB-completion. And then on the shell comes the output (on the same line) "error: key does not contain variable name: alias.b62c.." Pressing again TAB repeats the message. Even if TAB is pressed once, the printed text cannot be deleted with the backspace, so the only way to do something useful is to press ENTER.
While the command is indeed nonsense, in my understanding tab-completion shall do nothing, if completion is not possible. In particular it shall not print error messages and even the user types stupid things, he shall not be required to force ENTER to come back to a clear state, where a command can by types.
Greetings Dilian