Irony and Coincidence
Too many people claim something is the former when they actually mean the latter. For example, it’s not “ironic” that “Barbara moved from California to New York, where she ended up meeting and falling in love with a fellow Californian.” The fact that they’re both from California is a "coincidence." "Irony" is the incongruity in a series of events between the expected results and the actual results. "Coincidence" is a series of events that appear planned when they’re actually accidental. So, it would be "ironic" if “Barbara moved from California to New York to escape California men, but the first man she ended up meeting and falling in love with was a fellow Californian.”
Undoubtedly the most common mistake I encounter. Contrary to almost ubiquitous misuse, to be “nauseous” doesn’t mean you’ve been sickened: it actually means you possess the ability to produce nausea in others. e.g., That week-old hot dog is nauseous. When you find yourself disgusted or made ill by a nauseating agent, you are actually “nauseated.” e.g., I was nauseated after falling into that dumpster behind the Planned Parenthood. Stop embarrassing yourself.
If you’re looking for a practical, quick guide to proper grammar, I suggest the tried-and-true classic The Elements of Style, by William Strunk, Jr. and E. B. White. A few of these examples are listed in the book, and there are plenty more.