If two people try and break a block at the same time, when the block is broken it shows a shadow of the block for a second. This happens on the regular Minecraft client as well. (like the regular Minecraft client downloadable from minecraft.net) This happens while one person tryes to break the same block at the same time someone else is breaking it.
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I think I may have seen this before, and from what I understand, the cracks (not shadow) formed on blocks are per individual (1 set of cracks per person mining). If there is a slight delay, the cracks formed by other players will be visible for a moment once the block is destroyed. I cannot confirm this 100%, but I believe this is what you are reporting.
As stated in MC-1: "Block animation now disappears after 20 seconds"
so is this a bug?