For a while I was on the game paused screen, but when I came back I was going to get rid of a block of water. I tried placing a block in it multiple times. (The block I was trying to place was spruce planks, and the block under the water was also spruce planks. The default gamemode is adventure.) I was only able to place a block in it after I broke a block next to it and filled it in.
(I was in creative mode when I couldn't place the block.)
You cannot place blocks in Adventure Mode.
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