Yes in rare cases you can be denied a mortgage after pre approval.
You might be pre approved for a mortgage and then when you get approved the bank or mortgage company might find something else in your credit history or something else and then they may decide to deny your mortgage.
When getting a mortgage don't get too excited about the mortgage approval until you actually close on the house or property.
I've had a pre approved mortgage approved and then a few weeks alter got denied for some reason.
The bank didn't give me a very good reason about why they decided to deny the mortgage but I did go with a different mortgage company and bank and got approved and got the house.
So if you do get pre approved for the mortgage and then get denied then try a different bank or mortgage company.
Just make sure you have a good income and good credit score and good credit history before applying for a mortgage.