Here's a horror story for you. Before we went away on a break last week, I topped up the washer fluid in my VW T5. I used concentrated screenwash diluted with water from a container I store in the van.
Yesterday, I washed the screen and got hit with the worst rotten eggs/stinkbomb stench I've ever experienced.
I guessed that bacteria had grown in the washer bottle. We have commercial water testing kits, so I captured some in a plastic tray and tested it.
@woodpunk
There was an episode of Risky or Not (podcast) about risk of contamination of refillable hand soap dispensers. The risk I remember was from diluting the soap.