You do not type username, you should type your MineCraft name, that is why ti is saying he user cannot be found.
I was running a clock with the following command "testfor @p[score_Health=1,score_Health_min=1]" in a command block with a comparator leading to another one with "/particle mobappearance ~ ~ ~ 1 1 1 1 1." I was on single player. I have 4 gigabytes fo ram. I allowcated a gigabyte to Minecraft. Around 34 frames per second. The "testfor" command block was powered by a comparator clock.
Yeah, I know. I did a search but did not find it. I found it after uploading.
That is intended behaviour.