In over 15 years of travel & over 1 million airmiles, there are only 3 situations I can call scary. All 3 were on UA and 2 of them were on brand new Airbus equipment.

1. Approach to land at ORD, I'm in a window seat and we're about 500 ft off the ground.
The plane suddenly pulls up severly and banks to the right with FULL POWER. Seems a plane had strayed onto our runway and another 15 seconds could have been major trouble. Then it took us almost 20 minutes to vector around and get a landing slot...no problem 2nd time.

2. New A-320 from ORD-DEN, I'm in exit row aisle seat, take-off was uneventful & as we're climbing there is a LOUD swooshing noise & trembling under my feet. I reallly thought the plane was in trouble...but as I came to senses I reasoned that it sounded like the landing gear door may have opened.
That is exactly what it was! The pilot said it was a new plane in service for only 4 days & there was no problem (at least for now). The doors closed and the rest of the flight was fine. Upon approaching DEN, pilot told us we had to do a low flyby of the tower so they can take high definition photos of the belly of the plane to make sure everything is OK. After the flyby, we had to circle for 1/2 hr. until the techies agreed the gear was safe to land on. The part that really hit home was seeing all of the emergency vehicles & foam trucks on the runway when we finally landed.
3. This one is for Jim_B-Another new Airbus, taking off from ORD. I was in FC and just as we seemed like the plane would lift off, the
pilot stood on the brakes for what seemed like an eternity. My seatmate & I looked at each other thinking there was no way anything good was about to happen. Seems an indicator light came on and they had no choice but to stop the plane. As Jim_B said, we also had to wait about 25 minutes for the brakes to cool and have maintenance check out the light malfunction. That one was the scariest!