I remember when we had to port to OpenVMS Itanium. Biggest pita was floating point conversion. Alpha used VAX float while Itanium used the IEEE standard.
I laughed the first time I saw the exit code "%SYSTEM-W-FISH, my hovercraft is full of eels".
I still to this day own a copy of "Writing real programs in DCL". Looks like a collectors item, hahaha!
I also somewhat miss the mainframe development. We had a fantastic (albeit homegrown) build system and architecture (thin client, thick server - all C++) that was hugely enabling. Was super slick and easy to build, you could easily create previews for product people to run and test. Really tight development loops... loved it.