Twice for me. First was a Reno Air MD-80 out of SJC. We lined up and started the T/O roll to the north. About the time we hit the "big bump" (SJC flyers know what I'm talking about), a really bad vibration built up and the PF shut us down before Vr. As we we exiting the aircraft, I quietly said to the Captain at the door "nosewheel shimmy damper?" and he kinda smiled and said "yup". All in all a non-event.

Second was more fun with an RTO. It was on an ATR out of Key West on AA. Probably just before Vr we had the brakes slam on and heavy reverse pitch on the props. We stopped real quickly (seat belts are great things) and pulled off the runway and shut down. I told my wife to expect a strong smell of hot rubber and brakes and a visit from the red trucks with the blinking lights. Also, we would need alternate transportation to MIA real soon. The cockpit crew came on the PA and gave a really good heads up about what happened (warning light), why we were sitting (really hot brakes) and why fire/rescue was coming out (in case of a brake/tire fire). After about 20 minutes, we were able to taxi back to the terminal and re-book.