2.) "That's not what irony means!"
We use "irony" to describe lots of things. You might say it's ironic that you finally saved up for a new car and then it got hit by a meteor, or that you spent all your money on a gold necklace for your girlfriend and she turned out to be allergic to necklaces. "But wait!" some guy will interject, appearing out of nowhere. "That's not irony, and furthermore you are fat and you smell!" Then he will disappear into whatever dimension these people come from.

Well guess what, bucko: irony has about a million meanings, and that's one of them. It's called cosmic irony, and it refers to when the cosmos screws you over because it is a jerk. On top of that, there's verbal irony, which is pretty similar to sarcasm. There's dramatic/tragic irony, which is when Oedipus is like "This chick is hella hott" and the audience is like "Nooo that's your mom!" There's also situational irony, a more recently-coined variant of cosmic irony, where something plays out the opposite of how you'd planned it (like if you buy a bear trap to protect yourself from bears, then realize that you are yourself a bear).

3.) "That's a split infinitive!"
People often use Star Trek to provide an example of a split infinitive; the mission of the Enterprise was "to boldly go" all around the galaxy and sleep with gross aliens, and "boldly" comes in the middle of the infinitive "to go." But—and this is a serious question—who cares?

It's also a rhetorical question, because we'll tell you who cares: random 19th-century writers who are completely irrelevant and also dead. We would list them, but there is no point, because nobody has ever heard of them. Here are some sources that maybe you have heard of: Fowler's Modern English Usage, Webster's Dictionary of English Usage, The Oxford Dictionary of American Usage and Style, and, for good measure, George Bernard Shaw. They all think the split infinitive is fine. Shaw, by the way, is often considered the second most important playwright in the English language; the first most important spelled his own name six different ways, so maybe being a grammar pedant is not the best path to fame and glory.

4.) "It's not octopi, it's octopodes!"
This point is about words in general and not just octopuses, as octopuses are probably not very important to your life, unless you are on some kind of octopus task force (in which case you would know that most people just say "octopuses" instead of trying to be fancy). If you got a bunch of grammarians in a room... well, you would regret it, but you would also end up with ten different arguments about the Greek/Latin/Neo-Latin etymology of "octopus."

If you are thinking "Oh my God, who cares," then good for you, because that is the correct answer. Being pedantic about grammar is pointless because language only exists to communicate things; we only learned to grunt "fire hot" to impart the knowledge that it would be unwise to play in fire. So if someone says she's nauseous, and your initial reaction is to go "Aaaaactually, 'nauseous' means 'causing nausea,' the word you are looking for is nauseated, and furthermore herp derp derp," then you deserve it when she barfs on your shoes. And besides, you would be wrong.

LOL. Do any of these bother you?