The walls were pretty close to right angles... the counter only missed by about 2" at the long end. In any case, a measurement across the room would have told me all I needed.

But in another project, there were walls on a hallway in a legal office that had strange 'spaces'as if for a bookcase or something. They wanted counters installed.. about 6ft. by2 ft.... 3 of them. Someone measured the back wall length of the space and the opening length.. and they were all the same. We took the finished counters down and they wouldn't fit.. because no one had measured the diagonals! The spaces were all trapezoids! No right angles at all.
That memory was useful for me in several projects in my home shop.