Foremost in those issues is the fact that you tend to get complacent and not closely read what's on the page when you finish. You do a quick scan, think "No little red squiggles, I must be good" and move on.
But you'd be amazed at the number of problems trusting Spellcheck can pack in. Why, if I had a dime for every time a "soundalike" word or visually similar word snuck through and showed up in a manuscript I was editing (or - unfortunately more often - an email I was sending out), I'd be as rich as that guy with the castle in California.
Sometimes, though, people are just moving too fast, and so they push things through, ending up with sentences that might be interesting - but probably weren't intended. With that in mind, I offer you this gem I found recently:
"You will never see a Hearst pulling a U-Haul behind it."
While I'm fairly certain that William Randolph Hearst - or Patty, even - never drove anywhere pulling a U-Haul, my guess is that the author of that quip meant it to be a "hearse."
Don't worry, though. We caught it early, so it certainly won't be the death of him.