So, I was playing around with OutpostHD running numbers and checking out the simulation and was moving merrily along. At turn 269 I decided I needed more storage tanks. Opted to modify a tube section so I could add them to the current bank of storage tanks I had. So I bulldozed the tube sections I wanted to replace, hit the next turn button and everybody died.
I forgot that my Fusion Reactor, the colony's only source of power, was at the end of that line of tubes.
(https://snag.gy/UyZpFr.jpg)
Moral of the story?
Check what's at the end of the line before you disconnect a section of the colony.
The other moral of the story?
Redundancy isn't always a bad thing.