Off the top of my head:
-333 vice 158 vice 321 on a Leeds-Doncaster on a Sunday a few years back. Right farce that was!
-2+3 HST in Cornwall. Ok, that's just a Castle set minus a carriage. But the acceleration would have put plenty of EMUs to shame! Not to mention I was expecting a 150/0 when I saw the PIS displaying 3 coaches.
-Ok. This is pushing it a bit. July 2014 saw TPE charter in a pair of 47s and 8(?) Mk2s to run scheduled passenger services during the Tour De Yorkshire weekend. What a riot that was!
-A pair of Voyagers vice a HST was nice, especially when I found a carriage that "didn't exist" and shared it with just two other passengers on departure from Birmingham New Street.
-Back in March I had a 4 car 755 vice a 90+mk3 set on the last Norwich-Shenfield. It was swapped at Ipswich, where I alighted anyway. Can't be too many people that have boarded a 755 at Diss.
-Pair of 142s working a Leeds-Skipton stopper. (Though I know some jammy little **** who's had one on the S&C...)
-3 car 158 on the Penistone line back in the days of pacers being nailed onto it. 158 to Lincoln as well when that was rare.
-7 car 222 on a Liverpool diagram on Grand National day.
Easy nomination for the worst: Saturday lunchtime, Northern strike day. Rugby game in Edinburgh, Glasgow services either side cancelled due to earlier disruption. 3 car 185 vice 8 car 350. Thankfully I wasn't travelling on it, but it looked... cosy. It appeared to leave about 10 people behind, which was far less than I was expecting.