I poisoned my boyfriend.
We had been dating for several months and I asked him over for dinner with me and my two kids - something he had done a few times the previous month. He hemmed and hawed. I asked what was wrong, and he confessed that each time he ate at my place he got sick. He then proceeded to grill me on how cold I kept the refrigerator, how well I washed the dishes, how I handled clean dishes, where I purchased my food ingredients, etc. I wasn't amused.
But he was getting sick after eating at my place, so I looked back at the menus. Each meal had been entirely different - except that dessert had always included something made with chocolate chips - brownies, cookies, bars, etc - in deference to my kids' tastebuds. A lightbulb went on and I searched through the business section of the newspaper; the local Nestle plant (it might have been Hersey's - I can't remember) had been shut down for several weeks for salmonella contamination, and had just reopened two months previously. The article reported that some customers were complaining that management had reopened the plant too soon because they were still getting ill after eating the chocolate products.
The majority of people (including my kids and I) were not affected, but my boyfriend had recently finished three weeks of antibiotics after a work trip to a country where he shouldn't have drank the water, so he had nothing left in his gut to protect against any lingering contamination.
I replaced my supply of chocolate chips with the rival brand, which resolved the problem.