Holding a hot pipe (including a ghost) from the Hot metals mods (https://mods.factorio.com/mod/hot-metals) in the cursor and attempting to swap to the underground layer causes the following non-recoverable error:

The mod Actual Underground Pipes (0.1.4) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event the-one-mod-with-underground-bits::tomwub-swap-layer (ID 260)
Unknown item name: tomwub-hot-pipe
stack traceback:
    [C]: in function 'set_stack'
    __the-one-mod-with-underground-bits__/control.lua:187: in function <__the-one-mod-with-underground-bits__/control.lua:120>

Does the hot pipe place any entities, or is it just a decay item?

It is a decay item, but you can still place it on the ground, it just gets replaced with a regular pipe when you do.

Hm, okay. I'll look into it.

For now, I've just made a check that disallows layer swapping when holding hot pipes. How often do you use them for building? If its rare then I just won't allow it, which is a whole lot easier. Otherwise I'll properly support the hot pipes, which could take a while.

Very rarely, yea. Just preventing the game from throwing an error and possibly losing some progress is fine for me, personally.
Thanks!

No problem!