I have created method
@Inject
public String getX( @Named String a)

{ return a; }
which was the last method in a class file.

Then I manually reformatted it to
@Inject
public String getX(

@Named

String a) { return a; }

}
with 10 empty lines instead of 1 left here for readability.
Then I deleted each set of spaces by selecting it all and using Delete button.
Finally, I started quickly repeat Undo-Save-Redo-Save with fast keys. Several rounds resulted in the exception.
That kind of intensive 'work' may result in other exceptions as well.