Lenders get skittish when your life circumstances change during the period between applying for a mortgage and closing on it. Here are some things that you should not do (if you can help it) during those critical weeks:
Change employers. If you get a job with a new employer before you close on the mortgage, you will have to start over with the loan process. And there will be a strike against you for not having held your current job for at least two years.
Buy a car. If you pay cash for a car, the mortgage lender will want to review your finances to see if you will have enough cash in reserve. That could slow down or even kill the deal. If you get an auto loan, the lender will have to run the numbers again to see if your total monthly debt payments are still affordable. This advice applies for any big purchase.
Accept a big wad of cash. If your parents want to give you a few thousand bucks as a housewarming gift, don't cash the check until after you close on the loan. Otherwise, the lender will wonder whether you accumulated your savings on your own, and whether you're living beyond your means.
Become disabled. This is largely out of your control. But you can choose to drive safely and put off your skydiving adventure until later.