What do u do when you forget to trade champs in champ select?

Play...

1: Email riot immediately 2: blame Obama

I mean, you and the other person both forgot, so it's not really one person's fault. Just play it out and have fun with it and hope for the best.

I did this with a friend once. I picked him Ali for jungle (this was a long time ago) and he picked me top Kennen. I ended up playing smiteless jungle Ali while he played top Kennen with smite. I had him leash for me with his smite, and drop back into the jungle a couple other times to smite things for me so i could actually clear halfway quickly. We ended up winning the game.

say "o crap my bad LOL" and play it out.

That is how I got my poppy win in ranked this season :). Dude wanted me to swap him poppy, but never traded. I had never played new poppy and only played old poppy a couple of times. I also am a pretty poor top laner. In the end, I held my own and my team got fed and we ended up winning. I have also forgotten to trade people as well and have lost game because of it. It sucks but can be a funny moment. Just have to try and make the best of the situation.