tl;dr: Siloxanes—inert silicone compounds found in deodorants, lotions, and wipes—evaporate from astronauts on the ISS, get hydrolyzed by space radiation into dimethylsilanediol (DMSD), and contaminate the recycled water supply, fouling filters, heat exchangers, and the Sabatier reactor. NASA spent years identifying the culprit (even destroying gas chromatographs whose siloxane tubing kept contaminating samples) and has only partially mitigated it via hybrid HEPA/charcoal air filters, which caused a mold outbreak. The saga illustrates how mundane "unknown unknowns" and unsimulatable closed-loop interactions make life support engineering brutally hard—and pose serious risks for future Mars missions.