I'm working with PE8. Occassionaly, lie 5 minutes ago, I use the garbage matte (in this case the 4 point) and it will still leave parts of the scene showing outside of the box. Well, as I was writing this I realized why - but thought I'd share anyways. I was also using the green screen chroma key effect. The problem was I had the garbage matte on the layer below the green screen effect. Switching them around fixed my problem. I guess I didn't realize the order of the layers was important ;-)
OK - I'm not as smart as I thought I was. It didn't make any difference after all. As soon as I rendered the scene, the garbage came right back. Help???
UPDATE: This is not a problem with just the garbage matte - I get the same result with the crop effect - and doesn't seem to be related to the green screen effect either - I still get the same result. Very frustrating - this is now crippling my ability to complete this project :-(
UPDATE 2: Decided it might be time to upgrade to PE9. Looks like that has resolved my problem, but wow - the difference between how unrendered HD footage plays is night and day - extremely smooth.