I suppose starting INKEY's unquoted prompt with a / is a no-no (trying to make an option out of it).
But this one, first observed by Stefano Piccardi, causes no error message, high CPU use, and is uninterruptible:
Either of these (below) sort of work except that the prompt isn't seen until **after** the mouse click!
inkey /m /x /prompt/ %%key TCC: (Sys) The parameter is incorrect. "prompt/" Usage : INKEY [/C /D /K"keys" /M /P /Wn /X] [prompt ] %%varname
inkey /m /x /%char/ %%key
inkey /m /x ^e %%key inkey /m /x ← %%key (Esc entered with Alt0255-Esc)