I've posted a bug report (http://www.kdenlive.org/mantis/view.php?id=3123) but in case anyone in the forums can help out, I need some advice!
All files in my project show in the timeline with severe aliasing/jagged edges - this occurs with clips under .PNG titles (like the one pictured) and also on clips alone, with no effects.
When I render sections of the timeline, video is blurred, titles are extremely blocky and incorrectly scaled.
The project was fine for two days of editing, both preview and export worked normally (strange title distortion notwithstanding - thaough that's another bug for another bug report!) but when I started color grading, all of my clips in the timeline became jagged/aliased. Clips when viewed in the clip monitor are fine (see pic)
I didn't knowlingly change anything, though I removed some proxy clips around the time everything went wrong.
At the same time I found that all clips/PNGs with the Pan and Zoom effect had their scale and location values quadrupled. (i.e. X position changed from 100>400, scale 110 percent to 440)
This appears to be a project-specific problem - old projects are fine, other NLEs handle the same footage without errors, and this project shows the same errors on a fresh kdenlive install on another machine.
The project is utter chaos and has to be finished today - for lack of a better idea, I'm currently manually recreating my edit in another program. Better ideas very welcome!

clips in clip monitor preview fine, though not in project monitor.

Actually one of my older versions of the project still works, so I've retracing my steps in Kdenlive (saving plenty of duplicate versions with incremented names along the way in case it happens again).

finished video, with no aliasing:

Design Modelling with Jude Pullen and HowDo from Sam Muirhead on Vimeo.

still have no idea what caused it or how to stop it happening again.