Have you made any newbie mistakes when you were taking photos? or some newbie mistakes you saw other people do when they are taking photos?
Horizon in the middle . . . . .
using camera flash when taking landscape photo at night!
haa that's a good one!
I probably have done that a few times before!
but now with digital cameras, you normally look at the LCD or DSLR's optical viewfinder when taking pictures so chance of that happens is much smaller

Forgotten to adjust ISO.
Shooting bright sunlight at ISO1600.
Wrong ISO for me too.
Did a whole outdoor car event at 800.
haa i think i've done that before as well!
high ISO shooting outdoor daytime!
People pointing their speedlights straight up in to the sky.....
I get the ISO wrong quite often. Much less often with the D700 as it shows in the viewfinder.
The worst thing for me, taking the camera out and realizing there is no memory card.

arriving on a night shoot without my remote trigger ... only happened twice though and there's a work-around, but I still hate it when it happens. And of course, forgetting to change the ISO setting back to 200 ... but this one exposes itself after looking at the first results ...