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When I'm a complete truth-teller, you're happy with me
When I'm a complete liar, I'm still doing my job very well
When I'm mostly a liar, you know there's something seriously wrong with me

Who am I?

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1  
Probably not the intended solution, but could you be a princess? – GOTO 0 Jan 30 at 11:46
2  
I immediately thought a lawyer. – user18837 Jan 31 at 3:29
up vote 16 down vote accepted

You are

a clock

because when I'm a complete truth-teller, you're happy with me:

Accurate clocks are a good thing.

When I'm a complete liar, I'm still doing my job very well:

A clock that is as off as possible (i.e., 12 hours), is still very useful for telling the time since we don't usually need a clock to know whether it's AM or PM.

When I'm mostly a liar, you know there's something seriously wrong with me:

Any clock that's neither accurate nor exactly 12 hours off does not function as a useful clock.

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9  
I would say "complete liar" is a clock that keeps accurate time, but is not set to the correct time. The only way the time can always be wrong is if the hands advance at exactly the right speed. If the clock gained or lost time, it would occasionally have the correct time ("mostly a liar"). – GentlePurpleRain Jan 30 at 6:08
    
I object... why "complete liar" imply "always off by a constant amount"? Can't you have a clock which wobbles unpredictably but which advances on average at the correct rate so that it is never actually correct? – Mehrdad Jan 30 at 7:46
1  
@GentlePurpleRain is correct about my thinking behind the "complete liar clue". – Big Black Box Jan 30 at 9:08
1  
@Mehrdad no not really, clocks like the one you're describing don't exist in reality so it's a purely hypothetical possibility, whereas the possibility of a clock that functions properly but is set to the incorrect time (say five minute early) and still serves a useful purpose is a very common thing. – Big Black Box Jan 30 at 9:11
    
@user1717828 your third clue explanation is also a bit off, mostly a liar would mean it's sometimes telling the truth. – Big Black Box Jan 30 at 9:13

You are

A Man / Boyfriend/ Husband

When I'm a complete truth-teller, you're happy with me

You appreciate my honesty

When I'm a complete liar, I'm still doing my job very well

No, you're not fat

When I'm mostly a liar, you know there's something seriously wrong with me

You're not fat, just a little bit chubby

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You are an

answer verifier

when you tell the truth all the time,

we know that the answer is true or not.

When you lie all the time,

we again know when the answer is true or not. When you say it is "true", we know it is not and vice versa.

However, when you lie sometimes,

you are useless and we have no idea if the answer is true or not.

Although,

You may be a fortuneteller based on the same reasoning.

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I was gonna awnser this question again as a computer / boolean statement, but then realized your awnser covers the same reasoning. Might be worth considering expanding your answer ;) – Oak Jan 31 at 18:35

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