How can I get back at them
I cannot tell you enough how much i hope i never drink broken glass or have to think much about it ever again
Hate to the the bearer of bad news, but the average person typically consumes 400~ spiders and upwards of 1kg of glass in their sleep every night. If you've ever woken up with a mouth full of glass, that's why.
That one guy evening it all out by eating an unholy amount of spiders and glass everyday .
Title
NOTE : The word "unholy amount" is used because I'm lazy to do the math
It's OK I'm sure you will be fine so long as you are not the kind of person to smash a pot on the counter, break the spout and then act completely oblivious to the fact it is broken
Unfortunately since you have offered them bread and salt, their persons are protected by guest right
You have to plan their downfall after they have left
It sounds like your house guest gets back at themselves pretty regularly. I'm sure there's nothing worse you can do to them that they won't eventually do to themselves.
Tell them the coffee pot was being used to incubate a highly virulent form of STD and you were storing the specimens that survived in the vaseline jar. Tell them you're sorry it's come to this, but you required by law to inform anyone who may be exposed.
They may want to return the vaseline jar unless they want to be legally liable for the consequences.
And if they don't want you reporting them to the public health authorities, they may want to replace the coffee pot, since you are required to return the lab equipment and file a report.