I slept in a 4 season tent last april, -20F bag - had to unzip it several times to cool down, t-shirt only.
didn't look that warm oustside in the morning
One September, I spent the night at trail camp in a 3-season tent and 25F bag - one of my coldest nights in the mountains ever. Bag was covered with ice and the down had more or less collapsed.
Mousie was there and he wasn't too happy with my gear choices!
One day I'll get a bag that's in between these two - something like a 0F bag which would be ideal for anything but the worst weather up there.
