i.e, suppose you wanted to check whether the nearest player was standing on grass or not. One way that would make sense would be a command block with the command "/execute @p ~ ~ ~ testforblock ~ ~-1 ~ grass". However, any comparators sticking out of a command block will be powered no matter what: the game only checks whether the execute command succeeds, not the testforblock.
Linked issues
is duplicated by 4
MC-48967
when using /execute and /testfor, comparators determine the success of /execute and not /testfor
Resolved
MC-49069
Testfor, testforblock, and testforblocks booleans are not returned to command blocks from entities.
Resolved
MC-49387
Execute Command not activating comparator even though the command worked
Resolved
MC-50652
Execute doesnt give a comparator signal.
Resolved
There is also a problem using /execute testfor. I had this command on a clock:
/execute @e[type=Zombie] ~ ~ ~ testfor @a[r=10]
but even if I was out of range, as long as there was a zombie in the world, the comparator always outputted