How to reproduce:
Have a 2 video track above each other, transparent PNG image clip and video clip, image above video indeed.
Transparency of the image clip is lost! It's got a black background now. this worked fine in 0.4.
This is a serious bug, transparent PNG are the only way hoe to get somem level of transparency to Kdenlive!
Full kdenlive rebuild (kdenlive_builder script) from SVN sources updated 16.6.2007 (most recent ffmpeg+mlt+kdenlive at that time)