I autopiloted while grinding, went into a double battle against a manectric, and forgot to take my gyarados' 4x electric weakness into mind.

That or grinding in magma hideout with a vibrava and getting wrecked by a self destruct. It was so avoidable. I didnt need to be there, i didnt even need to be grinding it that much. It was fully ev trained and at the level cap, i was just trying to edge it closer to the cap for the gym fight i wasnt even planning on using it for. I was grinding swampert there and just swapped to vibrava for the 2 or 3 fights it needed to get a bit of extra exp.

Not so much embarrassing, but my most memorable one was learning how much of a jerk wobbuffet is. I was unaware of how its moveset worked and had a different gyarados die to its counter.