Minecraft shield on strike?

I recently played Nohitdealy on the Gommeserver, when I noticed that whenever I hit and miss, a sign suddenly appeared, which I didn't get to see. Does anyone know what I posted?

Do you use mods / texture packs / shaders? If so, it could be because… Maybe the file is corrupt

No, I'm playing the original Minecraft.

Okay then try to uninstall Minecraft so don't just delete it and empty the trash and then reinstall it. Minecraft could also be a little buggy