The problem seems to not be your command, but your clock.
With a comparator clock the redstone dust it is directly feeding to will alternate between 15 and 2 power, and the next dust between 14 and 1. You need to go one dust further out to have a dust which will actually turn fully off and on.
As of 14w10c, this is possible using scoreboard values.
First, you need to set up a dummy scoreboard variable. I used "sneaky" as my variable, but you can really use whatever you want (as long as you change the commands as appropriate). To set this up, you just need to use the command /scoreboard objectives add sneaky dummy. You only need to do this once.
I had to set up the command blocks in a very specific pattern for them to work, as shown:
I think this has something to do with how command blocks handle firing off "simultaneously". The comparator in this setup is in subtraction mode, creating a "comparator clock" which pulses very quickly.
The command blocks on the left are your scoreboard related commands. The one on the bottom left sets "sneaky" to 0 for all players:
/scoreboard players set @a sneaky 0
The command block on the top left sets "sneaky" to 1 for all players with the stealth boots equipped:
/scoreboard players set @a sneaky 1 {Inventory:[{Slot:100b,tag:{display:{Name:"Stealth Boots"}}}]}
The command blocks on the right are your effect commands. They give players the invisibility effect, or take it away, as appropriate. The one on the bottom clears the invisibility effect from all players without the stealth boots:
/effect @a[score_sneaky=0] 14 0 0
The top one gives the invisibility effect to all players with the stealth boots:
/effect @a[score_sneaky_min=1] 14 30000
Best Answer
This happens when the command finds no entities that fit into all the parameters and conditions you entered - it's just a false output. For example, if you enter
it will always output an error (case 1 above). If you want to stop this showing, edit the command so that it "fits" at at least one entity or place an entity so that it fits the commands.
This command will aslo always output an error, but this time the error will always be
the entity UUID is an invalid format(case 2 above):On the other hand, if you type a command like so:
you will get a false output (
the entity UUID us an invalid format) only if the farthest entity from you isn't a Creeper (case 3 above).But, if your testfor command may theoretically target more entities, and doesn't, eg.
then you will not get any output at all (case 4 above).