yeah i do that alot too. I'll be taking some pictures on iso 250 or 400 of my dog running around like nuts and then i'll switch to my 50mm 1.8 lens for a close up picture of somebody and i'll forget to switch my ISO. Just have to make it a priority to check your ISO often.
Every time I turn my camera off, I always switch back to 200 from whatever I am doing. The same goes for other settings such as single/continuous shooting, AF/MF etc...
I also quickly scan my settings on the LCD every time I turn it on. Good habits to get into.
I just did that today!
The other night we were camping and I took some pictures at the campfire at night, ISO 1600.
Today I drove all through the countryside taking pictures of farms, churches, fields, only to get home to grainy pictures and thinking WTF??
Duh!
I used to do that frequently too. I usually remember to check it now, because of that error. It destroyed several otherwise excellent images. Frankly, I wish my camera would reset to ISO 100 every time I turned it off.