Here's a funny one - I had a pump replacement on a pad last year that had ZERO slack PVC between the pump's output and the filter. It went:
Jandy Union (Pump), Slip, 90o Elbow, JVA, slip, Jandy Union (Filter). And only about 1" on the pump input side. It went Spa/Pool - JVA, 1/2", Jandy Union.
So - the Stealth II motor went out, and the customer wanted the 2-speed stealth as a replacement.
I pulled the pump (contorted to get it out) and got it into my shop. The new pump arrived and the unions were in the EXACT same place - WOOT! WOOT!... right?
We, I'll be danged - I looked and the threads were different! I couldn't believe my eyes! It seems some product manager in their infinite wisdom at Jandy decided that their new "VersaPlumb(TM)" thread was better than the NPT thread they used to have on their unions. So, I had to cut-out almost all of the pumping on the pressure side and rebuild it - all because Jandy changed the thread on the wet-end!