Today was an object lesson in how important that pre-ride checklist is. I took and from home and thought that the bike felt kinda "draggy". Sort of like the rear tire didn't have enough air, though I pumped up just before leaving.
Got home after 30 miles. Decided to lube chain, check brakes, etc. Noticed that the center bolt holding the rear brake on had worked loose and the pad was pretty tight to the rim. No wonder the bike felt slow today! 30 miles felt like 40, and now I know why.
Lesson learned. Always re-check the brakes before taking off.