The phrase describes mayhem caused when something is recklessly thrown into the intricate gears and workings of a machine. In the UK, it's a spanner. In the US, it’s a wrench. So for example, I was planning a world tour of stand-up comedy shows, but then the COVID-19 pandemic threw a spanner in the works. To spend a penny is a polite euphemism or phrase that suggests something else. The phrase goes back to Victorian public toilets, which required users to insert a single penny in order to operate the lock. Women would announce they were going to spend a penny as only women's public toilets required a penny to lock.