Pretty self explanatory. You cannot place a banner/sign on snow unless it's a full block of snow. Note that pretty much every other block can be placed on snow, and would destroy the snow.
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Comments 5
I did not know about the signs. Thanks for adding it. Also, it's not on just the first layer, but on every layer until a full snow block, if that's what you meant.
... nor do signs.
Confirmed.