**System Information**
Win7/64, AMD HD 6850
**Blender Version**
Broken: 2.70a. Already broken in blender-2.70-25ad7b4 (8th of March). Maybe broken before, but I don't have these nightlies anymore.
Worked: 2.69
**Short description of error**
In the VSE, if you select the channel in the toolbar of the preview window, the preview remains black.
**Exact steps for others to reproduce the error**
# Open VSE
# Drag one movie strip to channel 1
# Drag another movie strip to channel 2
# Select both strips and add a subtract effect strip
# If I want to see //only// channel 1, I go to the the preview window and change "Channel: 0" to "Channel: 1". In 2.69, the movie is displayed whereas in 2.70a, the preview remains black.