An Irken Defective is any Irken fitted with a faulty ID PAK. They cannot be brainwashed or controlled by Imperial Control Brains, and are therefore considered unpredictable and dangerous. Most "defective" Irkens are sentenced to death or menial labour, such as Janitorial work or serving as a Food Service Drone.
The Trial, one of the many unmade episodes, reveals that Irkens must go through an Existence Evaluation, in order to prove that not only are they not defective, but that they are useful to the Irken Empire. All defective Irkens have their PAKs removed and their knowledge deleted, so as not to contaminate the vast stores of knowledge that are programmed into newborn smeets.
Zim is the most infamous defective Irken, and is also likely to be the most defective as well. The only semi-normal section of Zim's psyche is his devotion to the Tallest. However, it can be said that not even that is particularly normal, as Zim's devotion appears to be many times that of a normal Irken.
In the unmade episode, The Trial, it's shown that the bad data stored in Zim's PAK is too much for even the Control Brains to handle, driving them insane, and declaring Zim the Best Irken Ever. This is something that The Tallest were extremely keen to avoid Irk knowing, but the Control Brains let Zim drive The Massive for ten minutes.
With the Control Brains being their superior, the Tallest could not refuse their order.
Although not official, there is plenty of speculation that Tak is defective as well, given her individuality in wearing unique clothing items, possessing a differently styled ship to typical Irken spacecraft, and even altering the Irken symbol to fit her own style.
Her personality is quite wild and erratic, similar to Zim's. However, Tak appears to be more calculating in nature. Also similar to Zim, she abandons the post that the Control Brains assign her (janitorial drone) in favor of pursuing what she feels she's meant to do: become an Invader. Some suggest that her skill is also evidence of this. All other Irken Invaders seen previously to Tak's invasion of Earth have very poor disguises (e.g. in the beginning of Planet Jackers when Zim is checking their progress) whereas Tak blends in perfectly, using a hologram to make herself look exactly like a human girl.
While this seems to be an improvement on standard Irken behavior, it's still deviant from the norm, suggesting that many 'defectives' are simply different, and not necessarily 'worse' than normal Irkens. There is no doubt that Tak is far more successful at being an Invader than Zim, leading to the conclusion that being a defective does not affect an Irken's intelligence.
She nearly destroyed Earth within days of landing, giving herself great advantages by hypnotizing an exceptionally wealthy businessman of hotdogs into believing he is her father, which gave her the resources to build her planet-destroying base. Only with the help of GIR, Dib, and Gaz, did Zim defeat Tak and save the Earth.
Bob's contempt for the Tallest is extremely abnormal, even for most defective Irkens. He also holds a low paying and humiliating job as a Service Drone, indicative of a low social caste, though this may also be because of his short stature.