• Create a new model
• Pop out the code tab to a separate window, side-by-side with the interface/widgets tab window.
• Add the below code to the code tab, where vvvv is not defined. Click Check and see the error as expected.

to go 
  set vvvv 10
end

• Click to the interface/widgets window and back to the code tab window (may not be necessary, but is how I did/do it)
• Enter a line break after the set vvvv 10 line.
• The UI should now perpetually bounce back and forth rapidly between the interface tab/window and the code tab window - like it's repeatedly trying to compile/check the code or something. The UI is mostly unusable at this point, but you can click the "pop out code tab" checkbox to put the code tab back in the main window and that'll stop the loop.

Sometimes I have to click back into the interface/widgets window after adding the line break to get the loop to occur, so it's not totally deterministic, but it is also not hard to force to happen.

Ubuntu 24, NetLogo 7.0.4. If reproducing doesn't happen on other systems I can get a video or more info, just let me know.