So the consensus is: Feel free to worry about whether or not you will actually close for the next month
Just hang in there OP - I am on my 2nd resale purchase. I was worried up until it passed ROFR (today!!! yeah!) and right after my excitement and relief waned I started to worry about closing

It's silly, but we're human after all. Could something happen to thwart your purchase? Yes (those things mentioned by other posters). Is it likely? No. ROFR is the REAL hurdle. Just relax and keep on top of the deadlines and call your agent if things are not on time. Good luck!
Terri