If you switch a pump from clean to contaminated water with a hydrolyzer attached, the hydrolyzer will happily continue producing oxygen. Normally a hydrolyzer fed with contaminated water will not run.
Occured as a result of this bug:
Connect a pump to a hydrolyzer. Fill the pump with clean water and let the hydrolyzer start working. Now fill the pump's reservoir with dirty water. The pump will stop pumping. Reload the game to get the pump started again. When the dirty water reaches the hydrolyzer, it will continue running and producing oxygen.