I had a scary one when I was taking off from Lima, Peru a few years ago. Immediately after takeoff the plane went significantly lower in power and we started losing altitude. My buddy and I looked at each other and both of us thought we were going down.
Shortly after we powered back up and went to normal cruising altitude and continued on with the flight. Two hours later the pilot came over the intercom, "Some of you were wondering what happened after takeoff. The tower had us take off directly at a landing plane so we had to descend quickly to duck underneath it." Scary stuff.