Rapid clicking possibly causes more of the item picked up to be recieved
BUG: Basically clicking multiple times on a block that belongs to you quickly enough may return two or three instead of one block. While I was considering how big to make a square of grass I was preparing for an idea, I was picking them up by clicking a lot on the ground so I don't have to click each block one by one... however when I looked at my inventory, I would notice that I ended up having a higher number of grass than what I had bought earlier. I then tried it again with just a line of 10 grass blocks and, through rapid clicking, got only two extra blocks. I am not sure if this could be repeated with other objects, but it might be wise to possibly fix this if that's the case

Need to fix; it won’t actually give you extra blocks, but it will appear to until you attempt to place them (or a relogin fixes it).
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Anonymous commented
Yeah, it does count wrongly if you rapidly pick up blocks. Can be quite annoying.
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Lance Dennison commented
And now, after legitimately buying grass and picking up grass with a single click, it seems the block count in my inventory got lower, possibly meaning that it was counting wrong. It's still probably a good idea to check on this bug, just to be certain