When the lavatory runs dry, it may produce extremely cold polluted water. It seems as if it was trying to use the temperature of water AFTER use, which could be a problem if there was no more water there.
This super cold polluted water will solidify and break the pipe as soon as it leaves the lavatory. But it won't necessarily leave it immediately - it seems that some lavatories may completely turn off when there is no input and won't even output their polluted water in such a case.
Steps to Reproduce
1. Build a lavatory 2. Feed it some water, then cut water off 3. Have duplicants use up all the water by using the lavatory 4. The lavatory should have 11.7kg of very cold polluted water in it now
Status: Known Issue
We are aware of this issue and have it logged internally but either we do not have a fix ready yet or we are not planning on addressing the issue in the immediate future.