Gamemode switching commands such as

gamemode creative @a

will pass (conditional command blocks, comparators, etc.) even if the impacted player is already in that gamemode. This does not match with other ouputs of this command, such as whether the command block outputs the message "your gamemode has been updated" 

Steps to recreate:

-Place a command block with 

gamemode creative @a

inside

-Place a comparator reading it

-Go into creative, and power the command block

The comparator power on, when it should be powered off.