Use of "of" as an auxiliary verb instead of "have" (example: "must of wanted that . . .")
Use of "i.e." instead of "e.g." when the intended message is "for example." Both are of Latin derivation: i.e. stands for id est and means "that is" or "that is to say." e.g. stands for exempli gratia and means "for example."
One of the most egregious shortcomings is a complete and total failure to comprehend when and where commas should be inserted. Commas serve both to indicate verbal pauses and to facilitate comprehension by appropriately compartmentalizing portions of a sentence from each other. A prime example of the latter role, one that frequently is neglected, pertains to the use of commas immediately prior to and after parenthetical elements. If your memory is fuzzy with respect to this term, consider the following example: "Peyton Manning, the most heralded quarterback in Tennessee football history, led the Vols to . . ."
In this example, "the most heralded quarterback in Tennessee football history" is a parenthetical element. It could be eliminated from the sentence and it would still make sense. Its insertion, however, breaks the verbal flow of the sentence and, thus, requires the use of commas immediately prior to and after it.
For those of you who get upset about "grammar/spelling Nazis," understand that your inability to communicate with clarity and precision quite simply undermines your credibility in our eyes. If you can't use your native language competently, why should we give your opinion serious credence?