While on downwind final practicing touch and go the other day, did one of those things that give you a funny feeling in your bottom. My 140 which I'm learning in has a three lever quadrant in it, throttle,mixture and carb heat.Since my knowledge is some what limited on what other 140s have, not sure if they are set up like this or not. Anyway while doing everything to get ready after beaming the numbers I reached to turn on carb heat and instead pulled the mixture lever all the way back and just as I did it, I pushed it back up and regained power but engine was running rough. Thought that I might have flooded it a little,anyway CFI took over and landed just fine,stoped and ran it up to smooth it out which it did just fine. Lesson learned, look at which lever you grab. My question, is there a lock or detent that would make that impossible for the lever to be move by accident?