Homophones...

[cashew]

Segway - That two-wheeled scooter thing you see some liberal yuppies standing on when they zoom down a sidewalk.

Segue - Transitioning into a different topic/theme without stopping.

(Disclosure: I actually think segways are kind of fun looking and honestly wouldn't mind riding on one.)

Surely, surely people should know the difference between these two words by the time they're out of high school.

If you can't tell the difference between those two words, you're probably not qualified to be giving addiction intervention advice. Yes, I am equating your ability to distinguish between homophones and your medical qualification. Those things have a tendency to correlate.