The bug:

So I was following a redstone tutorial on how to make a retractable wall from around 2012 when I discovered this bug. The wall worked fine originally, but at some point (at least a year ago, judging by the dates on the comments under the video complaining about how it didn't work anymore) a bug was introduced that made the wall not work anymore. As far as I can tell, when the top row of sand falls and hits the middle row in midair, one of the rows breaks (I think it's the middle one but it's hard to tell so it might actually be the top row)

How to reproduce:

Follow this tutorial, flick the lever to extend the wall, then flick it again to retract the wall (and thus trigger the bug)