Happened to me almost 10 years ago. Dropped my pannier bag and didn't notice until a few blocks later. Went back and found it in the middle of the road, having been run over, of course. Laptop smashed, backup hard drive smashed. Good thing I had the important stuff backed up elsewhere and I was also able to ($300 later) recover the contents of the laptop drive. My PhD dissertation was on that drive, and I was able to graduate. Makes me wary of ever commuting with a laptop again, though. Now, essentials like wallet, keys, phone go into a front handlebar bag.