Disclaimer: this will be filled with vitriol, and harsh language.
If either of these things will make you whine like a little bitch, you should have stopped reading one sentence ago.
"When you assume, you make an ass out of you and me."
What the hell is that supposed to mean? People make millions of assumptions every single day with no ill effects - when you wake up, you assume that nobody has released poisonous gases in your room, and so, you breath. When you eat, you assume nobody's poisoned your food. When you go anywhere not surrounded by eight inches of rective armor plating, you assume that nobody has a fifty calibre anti-materiel rifle pointed at your skull.
Everybody makes a million billion assumptions, and nothing bad happens. Why, then, does this phrase exist?
Answer: it's an escape clause. If you're too much of a whiny little bitch to grow a pair of balls, and admit that you fucked up, you blame it on false assumptions. If you're too much of a whiny little bitch to grow a pair of balls and tell somebody else that they fucked up, you blame it on false assumptions. If you're trying to be an annoying little bitch and piss somebody off (namely me) then you spout this piece of shit, meaningless phrase.
I'm not an angry guy. I know, I haven't given that impression here, but this is something that just bothers me, you know why? People hide behind it. Moronic, non-planning people use this to hide their incompetence by making others' failures seem like such a mind-numbingly simple mistake that whatever that idiot has screwed up in the past seems like an actual problem rather than stupidity.
People use it to try to make themselves smarter. They use it to bring others down. They use it to make themselves bigger by comparison.
Nobody likes feeling stupid. Everybody does, at one point or another. If you have never felt like a complete and utter moron, even for a moment, then you are a complete and utter moron all the time.
When one starts a sentence with "I assume that-" and is immediately cut off by this niggling little piece of shit kernel of language, they feel like a moron. They feel as if they've just made a mistake so elementary that kindergarteners would know better than to commit it. Conversely, this makes the person on the other end of the conversation seem more intelligent, better, by comparison.
Prove your intellect by utilizing your intellect.
Don't hide behind this. Point out the flaws in the other's reasoning, reveal the issues behind their plan, but don't use some prefabricated excuse for a logical argument. In the words of Dara O'Briain, "Get in the fucking sack".
Sometimes, yes, assumptions lead to mistakes. This is the way of all things. The simplest, even, of assumptions - one day, you'll assume that nobody is going to step out from behind that van parked by the roadside and then you'll be proven wrong.
The way to prevent this?
It's not to eschew assumptions in all forms, to label them as some blasphemous, leperous pariah of reasoning.
The solution is: THINK!
If you take a moment to think about anything, then you will make fewer mistakes, and the ones you make will at least be legitimate.
In short, don't be a bitch.
Say that a specific assumption is faulty. Don't blanket them with the statement above.
Don't be a moron.