I cook on the fly. Half the time when I'm making dinner I'll switch things around according to my mood, my preferences and what is in the pantry. With baking I stick a little closer, but not by much. I really do try to stick with what is written, but sometimes I get stuck.
Like tonight, I am baking a cheesecake to drop off at school tomorrow, it's for some sort of teacher/staff appreciation thingy. I *thought* I had all the ingredients on hand and that's why I volunteered to make my basic cheesecake. Note the thought. I sort of had everything, I was just shy 1/4 cup sour cream. It's late, its cold outside and I just plain don't want to go. So I haul out my cookbooks to see how I can make do. It's sour cream and I've got the cream, how hard could it be?
It turns out not very hard at all. One quick substitution was 3/4 cup oh heavy cream with 3 to 4 drops of lemon juice. Add the lemon juice and let sit for 30 to 40 minutes. It basically worked and I was able to finish the cheesecake. Whoo-hoo substitutes! I get to stay in.