1: Why don't you watch where you're going.Generally used when two pedestrians collide with each other. It's meant to imply that the person you're saying it to wasn't looking where they were going and that you were… I mean it'd be exceptionally hypocritical for you to tell someone else to look where they're going if you were not doing it yourself.
Why does this make you an idiot?
Well if you were looking where you were going why did you collide with the person who was suffering the obvious disadvantage of not looking where they were going? What this phrase really implies is that you are prone to walking into people deliberately, like a fucking idiot.
2: I'll be with you momentarily
This one gets me every time. It's used frequently (especially in the retail, hotel and service industries) and I'd say about 99.9% of the time people use it incorrectly. In fact I'd say that this phrase has been misused so frequently, by so many people and for so long, that it may actually have to have it's definition redefined to match what people think it means.
Why does it make you an idiot?
Momentarily does not mean "in a short while" it means "for a short while".
3: I'm not being funny but…
Often used just before imparting some form of (generally unwanted) advice or opinion that could be considered offensive or insensitive.
Why does it make you an idiot?
People who say this, don't need to say it, as they never follow it with anything that could ever be misconstrued (even slightly) as being "funny". Rude yes, personal yes, inflammatory most certainly, but never, EVER funny. A common and annoying misuse of the word funny.
4: Can you fix my computer?
This one is especially irksome to me and it is something that I get asked often. I know how to use a computer so the reasoning usually goes that I can instantly tell what is wrong with any fucked up computer system (and usually over the phone), with the absolute minimum of information (probably described by someone who doesn't know how to describe what is wrong) and then be expected to direct them (again over the phone) how to make it all good and shiny and working again.
Why does it make you an idiot?
I know how to use the kitchen sink… but that doesn't make me a plumber.
I know how to watch a television set … but that doesn't make me a TV repair man.
I know how to drive a car… but that doesn't mean I could strip down its engine (actually I could but that's beside the point).
The Idiot's guide to getting things fixed:-
Sink broken + Call a plumber = Smart
Television broken + Call a TV repair man = Smart
Car broken + Call a mechanic = Smart
Computer broken + Call a writer? = Idiot.
5: Do you want another beer?
This really is the most idiotic thing you could say.
Don't be an idiot, just get me a pint of Stella and save your breath;)
