I was editing away tonight on a few commercials in Vegas v11 64-bit build 521 when a couple of my clips got replaced with another clip from the timeline. I had about 35 tracks, nothing too fancy, mostly color correction, pan/crop and track motion. The two clips that got replaced were DNxHD video with no effects or processing applied whatsoever. I've done a lot of projects in build 521 with no problems, this is a first. I was able to delete the replaced clips from the timeline and place the DNxHD clips back on the timeline via Explorer so it just took a few moments to correct it.
Anyway, it appears that the "replace bug" has Just thought you'd like to know...