My first call on my own was to change a #201 air filter. HO watched silently the whole time as I fumbled with the thing and ended up cutting myself too.
First actual service call was when I was on call (resi). It was a no heat at midnight, horizontal Lennox gas furnace in an attic. Ended up finding a bad limit/ fan control after about an hour and half. I remember I pulled the t-stat off the wall to jump it out before I realized it could be done at the unit lol.