When I connect a comparator clock, which is a fast clock, to a command block, sometimes the command doesnt get excuted.

Description of the photos: (This is an example)

I gave myself speed effect by potion, and then typed effect @p clear in the command block. This command should execute and remove my potion effect.
The redstone clock is running, but it doesnt do anything. My potion effect is still there.

Update: (Very interesting)
When I extend the redstone by one block, the command is successfully excuted, seen in the new photos. This is unexplainable using the proper redstone mechanics.
Also, the direction at which you place the mechanisms doesnt seem to affect the outcome.

Update:
The world that contains the bug is now uploaded. I last ran it with 14w02c.

Update:
Two things:
1. The bug is also found in 1.7.2 and 1.7.4.
2. At the exact moment when I put down a piece of redstone to connect the command block to the clock, it excutes the command once, but that's it. After that the clock is still running but the command block doesn't respond, no matter how much potion I drink.