1S1S projects that draw a line than attempt to sense that same line don't work unless there is a bit of a delay between when the line is drawn and when color sensing is employed. Try setting Delay Iterations to zero and watch the right-most square fall through the green line. The left square is drawn after the right one and apparently the line color can be sensed by that time. Note that in Scratch 1.4, this kind of project works without a delay of any sort between when the line is drawn and when the sprite can sense it.
