I'm ashamed to say this, but there have been so many times (as in, way way way too many) that after I put M down for his nap, I forgot to turn on the monitor (the parents' unit) in the living room. I'd be cooking or surfing the net or whatever, and then 15 minutes later (sometimes more) I'll be like, "I wonder why it's so quiet?" Then DOH! moment sets in and I quickly turn on the unit.
I know it's really NBD; ours is an audio monitor, and if M were going to cry, I would have heard him regardless of the monitor was on or not. But still, I always feel guilty when I forget!