Okay, finally I'm reporting what I consider to be a bug and that has bitten me so many times... (not hard enough obviously)
In one of my projects I do a timelapse scene from part of a H.264 clip shot at 25fps, by setting the speed effect to 1200% (12x speedup). This scene can be seen from 00:00:14 to 00:00:18 in this video: https://www.youtube.com/watch?v=cYHCrAF0vUI on YT. Around 00:00:17 the rendered frames freeze and are repeated. However, the original clip from which this scene is taken has many more frames, so there is no reason to repeat the same frame over and over again.
With the newest (Kdenlive/MLT) nightlies from 2014-02-17 there is an ugly interaction between the zoom and pan effect and the speed effect: it causes size and position to be miscalculated so I get a big black bar at the top; it looks as if this black bar is actually a part being clipped off without being told to do so.