Either ( 2 , 3 ) or ( 2 <SPC> 3 ) would have worked, but using both the comma and the space at once yields:
1: ( ... 2: ( ... 1: (2, ... 2: (2, ... 2: (2, ...
. 1: 2 . 1: (2, ... 1: (2, 3)
. . .
( 2 , <SPC> 3 )
Joe probably tried to type <TAB> <DEL> to swap the extra incomplete object to the top of the stack and delete it. But a feature of Calc is that <DEL> on an incomplete object deletes just one component out of that object, so he had to press <DEL> twice to finish the job.
2: (2, ... 2: (2, 3) 2: (2, 3) 1: (2, 3)
1: (2, 3) 1: (2, ... 1: ( ... .
. . .
<TAB> <DEL> <DEL>
(As it turns out, deleting the second-to-top stack entry happens often enough that Calc provides a special key, M-<DEL>, to do just that. M-<DEL> is just like <TAB> <DEL>, except that it doesn't exhibit the “feature” that tripped poor Joe.)