Haven't left the flaps down yet. But I did notice a rather anemic climb in a 172 a few months ago. Started checking throttle, mixture, carb heat, etc. Then I noticed we were on the left mag only. Doh. Didn't get back to both when doing run-up. Now its a second glance at mags after run up.
The one thing I was glad of about that situation and from reading what most people here said was that you noticed something wasn't right and started looking. I was mad at myself for being on wrong mag setting, but at least I didn't fly that way very long.