If you are in the map editor mode and deconstruct some pipes (above or below ground), then attempt to undo that deconstruction with Ctrl+Z, you get this error: (does not apply to ghosts)

The mod Actual Underground Pipes (0.1.18) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event the-one-mod-with-underground-bits::on_built_entity (ID 6)
__the-one-mod-with-underground-bits__/control.lua:420: attempt to index field 'item' (a nil value)
stack traceback:
    __the-one-mod-with-underground-bits__/control.lua:420: in function <__the-one-mod-with-underground-bits__/control.lua:391>

With or without 'instant deconstruct' enabled?

Both. It also only happens when the cursor is empty when the undo command is made, which I didn't realise initially.

Fixed for next release