If you have marked favorites, rejects or keyword ranges in the Event Browser and are filtering on those, it is easy to have a 1-frame sliver of a clip, which in the "filmstrip" view looks like a full clip. Then when you add that to the timeline it is only 1 frame. It is usually caused by your hand shaking or the mouse button bouncing when you mark ratings or keywords.
When you see the 1-frame clip on the timeline, select that and do SHIFT+F. It will show where in the Event Browser it came from. Look at the bottom of the browser and it tells you the duration of that clip. If it shows 1 or 2 frames, examine if that was a rating or keyword range or what, and fix it.