If you don't slit a pot pie before cooking it the steam in the pot pie can build up in the pot pie and cause the pot pies crust to crack, bulge or even collapse.
If the steam is not allowed to escape from the pot pie when you cook it the pot pie could even burst open and make a mess.
Always slit a pot pie before you cook it to avoid any potential mess.
There have been a few times when I have forgotten to slit a pot pie myself and it cooked okay but it also collapsed some but thankfully didn't make a mess those times.
However a few other times when I didn't slit the pot pie the pot pie splattered in my toaster oven and made a mess.
So I always try to remember to slit the pot pies before cooking to allow the steam to escape and for the pot pies to cook better without collapsing or making a mess.
When you cook a pot pie in the oven or toaster oven, the moisture within the pot pies turns to steam as the filling and crust of the pot pie heats up.
And without the steam being vented, the steam has nowhere to escape and then pressure can then cause the crust of the pot pie to crack, bulge or collapse and may even splatter in some cases.
A cracked or collapsed top crust on the pot pie can make the pot pie appear unappetizing although it is still okay to eat.
If the crust of the pot pie cracks or collapses, the filling of the pot pie can leak out and cause a mess in the oven.
Simply cutting the slits or other vents into the top crust of the pot pie before cooking it allows that steam to escape to prevent the pot pie from cracking or collapsing or leaking and making a mess.