What I expected to happen was:
When I attached my command block to a redstone clock and gave it a /gamemode command, it would change my gamemode as expected with the command /gamemode 2 @a

What actually happened was:
Nothing. I tried with another command (/say) and it worked fine.

Steps to Reproduce:
1. Make a 2-clock
2. Attach a command block to the output
3. Change the command block's command to /gamemode 2 @a