Taking a list that resembled something approaching a fluffy, characterful army that exists in the stories instead of the flavour of the month WAAC
For this reason, I will always be terrible at 40K
Oh my god yes, so much this.
Aside from that I've made more blunders than I can recall. At one point in early 8th I was using a lot of Khorne Berzerkers and I would routinely forget their second round of attacks.
I'm terrible at deployment and regularly sabotage myself before turn 1 even starts.
In 7th edition I played Chaos Space Marines vs Tau and was getting ANNIHILATED. Turn 4 came round and I remembered Cypher and 8 Terminators were still in reserve.
Another time in very early 8th I played a game against Grey Knights and forgot my Daemon Engines were indeed considered Daemons and played far too aggressively. I lost 2 Forgefiends and a Heldrake in a single psychic phase.
Possibly my worst offender though was in a 7th ed tournament. Early on I was paired up against a very competitive Dark Eldar player who regularly placed highly. I was off to a terrible start but managed to cripple a few transports and had a good chance at winning on objectives if I could clear some infantry. I subsequently forgot my Heldrake existed and played until turn 5 without it. In the 2 turns it was alive it destroyed a transport and incinerated 2 units of Kabalites sitting on objecives. If I remembered it earlier on I might have won that game. He went on to win the whole event rather solidly.