I just tried creating a project which relies on color sensing and noticed that while it does behave correctly in Scratch, it sometimes fails in the online version.

it's an attempt to let a sprite bounce off irregular surfaces:
http://scratch.mit.edu/projects/Jens/98418

It involves rotating a sprite until colored "sensor" dots touch a surface color (black) to approximate the surface's slope. Sometimes the black surface color seems to be sensed by the sprite despite it actually not touching the sensor. I also noticed that this bug appears more frequently the slower the sprite is moved or rotated (Try setting the bouncer's speed to 1).

Looking closely at the sensor dots in the Java player I sometimes seem to make out a dark "mask" around sprites. I wonder if  that may be part of the problem.

First I thought it might be related to image compression, so I tried uploading with image compression turned off - no luck. Then I remembered that there is a known bug in version 1.2.1 related to image compression, which cannot be avoided by unchecking image compression:

http://scratch.mit.edu/forums/viewtopic … 308#p14308

Hence I did as John suggested in that post and switched to v.1.1 to upload my project. But I'm still encountering the same problem, albeit less often than before.

Any advice?