In Justice Scalia and Bryan Garner's Making Your Case: The Art of Persuading Judges, they argue (quite sensibly) that if you want to be taken seriously during oral arguments of a case, you need to pronounce words correctly. What's the correct pronunciation? Generally, it's the preferred pronunciation, meaning "preferred by well-educated people" (p. 145).
For example, they argue that the six words listed above are among those that are commonly mispronounced. Chances are that you, like me (despite us being reasonably well educated people), have also been pronouncing at least a few of them incorrectly (or, at least, by their non-preferred means).
Here are the original six words, linked to their Dictionary.com definition (follow the link, then click the speaker icon for the spoken pronunciation):
comptroller [kuh n-troh-ler; not komp-troh-ler]
coupon [koo-pon; kyoo-pon is acceptable but not preferred]
err [ur, er; not air]
flaccid [flak-sid; although flas-id is also acceptable]
heinous [hey-nuh s; if there is a common mispronunciation, I don't know of it]
schism [siz-uh m or skiz-uh m]