The Bug
You can't place a sign on sugarcane, but you can on amethyst buds/clusters.
How to reproduce
Place sugarcane next to water
Place a budding amethyst and an amethyst cluster on top
Try to place a sign on the sugar cane
β doesn't work, as expectedTry to place a sign on the amethyst cluster
β it places the sign
I believe there is currently an issue with some blocks, like amethyst buds / clusters and candles, where they act as full blocks, and it is possible to place things on the sides. It probably shouldn't be this way. I would say this is related toΒ MC-203572, which pertains specifically to candles, but I think all the different blocks like this should be categorized under one bug.