To create this bug, first make a variable called "my" (make sure the letters aren't capitalized). Then take a "Set [var] to (val)" block and try to find your "my" variable in the dropdown list. You will instead find that the "my" list of properties of a sprite has jumped above the black dividing line, as though it were a variable.
While this bug can easily be worked around by using "Run [script] with inputs..." or with a sufficient understanding of "Split [script] by blocks", it could still be confusing especially to a new user. And to address why anyone would want a variable called "my", I discovered this because I was working with the simple variable name "my variable" and needed a second variable. So I just typed "my", and suddenly found that I couldn't easily use the "Set [var] to (val)" block anymore.
