For example, he could put the money into a conservative bond portfolio and by spending the interest and drawing down the principle [read principal] he could also spend $4,000 a month. (Richard H. Thaler, "Annuities and the Puzzle of Income," New York Times, June 5, 2011)
A good principle: Don't invest your principal on the advice of a newspaper columnist who doesn't know -ple from -pal.
"I can tell you that, if it was [read were] me, I would resign." (President Obama, quoted in Andrew Miga and Larry Margasak, "Obama says he would resign if he were [note correct usage here] Weiner," msn.com, June 11, 2011)
"There are three moods in English: the indicative, used for facts, opinions, and questions; the imperative, used for orders or advice; and the subjunctive, used in certain contexts to express wishes, requests, or conditions contrary to face. Of these moods, the subjunctive is mostly like to cause problems for writers." (Diana Hacker, A Writer's Reference)
Mr. Obama should have said, "If I were" Representative Weiner. That's because the subjunctive is used in if statements that are contrary to fact, as here: Mr. Obama is not Rep. Weiner.
Misuse of the subjunctive mood is common. "I wish I was [were] in Dixie." "If I was [were] to say to you, 'Girl, we couldn't get much higher.'" "If I fell [were to fall] in love with you."
Correct usage in popular music is rare, but not unknown. "Wish I were down on some blue bayou."
Mr. Obama deserves some sympathy here. He made the statement without the aid of his TelePropTer, which -- whatever its other faults -- generally gets the grammar right.