I noticed when placing panels (stone or wood) on a snow block they "float" after destroying the snow block. Normal minecraft blocks destroy the snow as they are placed.
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Not sure if this is just as expected with the panel code or possible bug. Just happen to notice it while building at my new snow site.
Ok, I just tracked this down, and it appears to be a bug with vanilla. When a block is placed over snow, its facing is set to be downwards (instead of upwards as you would expect). The thing is, since there are no downward facing vanilla blocks, this doesn't show up normally as a bug.
It would be a real pain in the ass to fix, either modifying a fairly low-level base-class (ItemBlock), or creating my own class to override that behavior. Neither solution seems particularly reasonable, so given how minor this bug is, I'm just going to leave it as is.