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).
Yeah, it does count wrongly if you rapidly pick up blocks. Can be quite annoying.
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