See, here's the thing, though: If it's so common, and you already acknowledge it being so common, why didn't you take extra care to avoid it? Trying to hide behind the statistic, that's not an excuse, that's an admittance of neglect. And trying to blame it on technological implements adds admittance of willful incompetence with modern consumer-level information technology.
"Doesn't affect the discussion" Ah, good old "you did understand me anyway" defense. Sure, it doesn't affect it. As does not washing your hands after taking a piss, as does speaking while chewing, letting your unfortunate listeners piece together your intended message from the mumblings between the flying breadcrumbs, as does sneezing without covering your face.
No, making you feel bad is not my goal. I just happened to make use of my freedom to not remain silent when people think they can get away with being lazy, discurteous snots. And if it makes you feel bad that you can't carelessly puke some text fragments into a public forum without consequences, or even just that you got a typo pointed out, then that sounds more like a "you" problem. Offense is taken, not given. Sure, some offense is more difficult not to take, but come on, you were neglectful, got called out on it, and decided it's better to whine about it instead of fixing it and moving on? Don't know about you, but, for me, whining stopped working in a discussion when I started learning to talk.
Yes, mistakes happen. Not giving a scrap how you present yourself in writing and at least trying to make an effort, however, is a conscious choice. And you're better than that, aren't you?
Speaking as someone who's
very particular about using proper grammar and spelling things correctly, I have to say that you're definitely the asshole in this situation. "It's" vs. "its" is definitely a common error to show up, and more often than not it's not a result of someone being lazy or not knowing the correct spelling, but instead is the result of a phone (or other device) autocorrecting as someone types or of someone fat-fingering their phone keyboard as they write.
I agree that proper spelling is important, and that we shouldn't let grammar fall by the wayside just because we're in a more casual context, but even so that still doesn't mean we need to be operating on all cylinders at all times.
In a setting like this, where it's essentially a somewhat more structured online chatroom, minor spelling mistakes (especially those caused by unwanted computer intervention such as autocorrect) are acceptable, especially if they don't obfuscate the meaning of the message itself. After all, it can be quite difficult to spot that apostrophe (or lack thereof) when typing out a quick message on a phone screen, so if the message passes the initial visual once-over then it's likely that the person in question won't see the mistake until they hit send. There's no need to jump on someone for this type of error in this type of setting. They certainly aren't being negligent in any way, they're communicating like a normal person in this setting, with all the minor errors and quirks that come along with that.
You have, and this is especially true of the quoted response, been unnecessarily rude and confrontational in your replies, acting as though the original error was some grave sin and the subsequent callouts against you for being pedantic were somehow deep personal attacks. You make this out to be some sort of horrible trend that's degrading the very foundation of our language, when the reality is that it was a simple, understandable mistake that the OP either didn't notice or didn't go back to correct because they aren't hypervigilant regarding their use of language. In the quoted message, you attack both their character and the character of their actions (something you do again in your interaction with captain_crunch), and you compare their mistake to acts that actively spread infectious disease.
I want to be clear here: To say that their mistake did not affect the discussion (until, of course, you brought it up) is 100% accurate. Any person with even a passing knowledge of the English language will automatically correct "it's" to "its" in their head when they read that message, especially if they're familiar with the quote. It's not at all similar to speaking while chewing, where a listener will have to piece together meaning from multiple mumbled or slurred sounds. It is not an equivalent comparison (and that's setting aside the whole actively spreading infection disease" comparison you made).
In the end, by calling out their mistake in the way you did, you helped nobody, accomplished nothing beyond making yourself seem pedantic, and generally made an ass of yourself (especially given the way you've been responding to subsequent messages).
As an aside: It's "discourteous", not "discurteous". Furthermore, "mumblings" isn't a word. The plural form of the noun "mumble" is "mumbles", and that's the only noun (other than "mumbler", which is clearly not what you're looking for here) derived from the verb "mumble".
Just figured I should let you know. Y'know, so you can stop being so neglectful.