I started a new job at a nice-ish office, next to a pretty fancy mall. On my 3rd day, my colleagues invited me to lunch at the food court of the mall. A couple hours later, I started getting nauseous and sweaty. Had to rush to the bathroom a couple times, which looking back I was lucky it was coming out of one end instead of two. After the 2nd time, I thought I'd chance it and rush home rather than having to go through another ~2 hours of this at the office. Luckily I lived close to the office back then so I was able to make it home. But in my rush I forgot to tell my manager, just told a coworker sitting near my desk I wasn't feeling well and left. I emailed my manager that night, telling him that I also wouldn't be able to make it in the next day. So my very first week at this job, I was only there for 2.5 days before getting sick for 1.5 days. Not the best first impression.

I didn't know until much later, but my manager told me that he thought I hated the job and was about to quit after a couple days.