Three times that I can think of off hand. Once I had been sick leading up to the race and may have been a full lap behind in the indoor 5k. Once when I was border line to get into a race and only found out when I didn't have time to warm up. I was not very happy with either race and that I was in last didn't help but it didn't make things much worse
The first time I was moved up into a faster heat in HS. I PRed and I think it was my first time under 4:50. I didn't realize I was in last until I finished and somebody said I was bringing up the rear. Since I'd just PRed I didn't really care.
If you think you are borderline to finish last treat it the same as you would being seeded first. Be aware of it and knowledgeable of your ability relative to the field then run your own race. If you are a 5:00 miler racing 4:30 guys try to run even splits and see if anyone comes back to you later. If you are a 5:00 miler racing 4:40 guys be ready to tuck in and see if they can pull you to a fast time.
Ultimately, the one thing you cannot control is your competition. If you run a good race and finish last it says more about the people you are racing than anything about you.