@futurebird I have a fun one! Something that likes to grow on (or is produced by something that grows on) temperature-abused cooked meat dishes. Eat a dish and it's fine, eat the leftovers that weren't fridged until a couple hours later than ideal, and there's a small chance I get anaphylaxis from it. Took years to figure out the pattern because there was no shared ingredient among the triggers!
I started being a real hardass about either keeping the food hot/refrigerating the leftovers promptly, and not eating meat leftovers unless I was confident that those had occurred (even at some risk of offending someone that I didn't trust their food handling). Still rarely got it from restaurants. Started vetting restaurants by checking to see if they had a pattern of food holding temperature violations on their health inspection reports and it's been a good number of years since my last reaction. Still carry epi-pens, of course.
Would love to know the actual trigger, but at least I figured out how to avoid it!