Terminal in error state after runny buggy code is apparently unusable afterwards

Hi

I have a question about terminal, when I run a class from it that contains an error I get a report of error as I would expect, and then an apparently random three letter code is appended to the namespace name in the prompt, as shown below.

GMMHTIE>do ##class(temp.Nothing).What()
 
 w !, tParisAnswer
      ^
<UNDEFINED>zWhat+23^temp.Nothing.1 *tParisAnswer
GMMHTIE 2d1>

Then, when I try to enter a command I get an error

GMMHTIE 2d1>do ##class(temp.Nothing).What()
 
DO ##class(temp.Nothing).What()
^
<COMMAND>^temp.Nothing.1
GMMHTIE 2d1>

Whilst the terminal is in this state, it is apparently unusable, and I have to close it and start a new session.

How can exit this state?