I've left it long enough, feel like I should post mine now.
I'll set the scene.
Arguably, my biggest game as L4 AR. Was last season, Southern League Premier Division, 2nd v 4th (both had games in hand on 1st, both were looking for promotion). Had been appointed to this game on merit having achieved very high AR marks through my 2 years as a 4. Absolutely buzzing with excitement for it, was over the moon. As you can imagine, we were being assessed.
11 minutes into the game and I flag for offside (I'm benchside btw). Ref blows whistle, awards IDFK and then all of a sudden, the away bench erupts with anger! Screaming at me, screaming at the ref, but still couldn't understand them. FK is taken and I get closer to the halfway line...to find out that I'd given it from a goal-kick! The biggest issue...? The striker was one-on-one with the keeper and was likely to score (as it turned out later in the game when he did so!)
To make matters worse, the assessor was sat directly behind me. I held my hands up to all parties (ref, bench, assessor etc) and apologised for the mistake. Club were acceptant of the apology, assessor & ref less so.
Assessment came through as a 66, ref got docked marks for it too and took him out the AA bandings and he ended up finishing 1 place below promoted refs.