When I have cheats on and am playing in creative mode, when I switch to survival mode, regular blocks like melon blocks and dirt blocks become unbreakable. I was able to break the melon block with a diamond sword, but iron bars were unbreakable and dirt blocks were unbreakable. I have to switch back to creative mode to remove them.
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Cannot reproduce.
While being in the "unbreakable" game mode: Please force a crash by pressing F3 + C for 10 seconds while ingame and attach the crash report here.

Duplicate of MC-3035.
Are you sure you didn't switch into adventure mode by accident?