I have a few, and it was always with Sam, the barn's Arabian.
I like to use one horses grooming halter to move the horses- slips on fast and you can go. Well, I didn't realize the grooming halter was too big for the Arabian... and I am leading him- and the halter slips off completely! Sam, being the good horse he was- just stops, and here I am walking on until I realize I am dragging the halter on the ground.... duh.... So glad the horse didn't run off or anything! Just stood there with a look on his face like he couldn't believe it took me a bit to realize I was just leading the halter anong and not him.
Second one- same horse ( different halter tho...lol) The lead line I was using was broken- didn't know it at the time- someone just tied the rope to the hook thing. GO to lead the horse out of the stall- lead rope drops off. The horse just stops, and I was like 'why you stop?' Oh, lead line broke.... I jnow plenty of other horses in their excitement to be turned out would have just went for it. Sam had a heart of gold and patience.
One other silly mistake- putting my Bitless bridal on the horse I lease- I didn't make sure the bridal was wide to slip it on easily, and the poor horse thought the nose band must be a bit to go in his mouth because of how the bridle was going on. Horse opens his mouth and worked to get the 'bit' in his mouth....and the result-had to take the bridle off- and get the horse to let go of the noseband.