Another quick and efficient way to vaporize your credibility with an audience is to make basic mistakes with pluralization.
A few examples:
Neither of them were at the concert. (Neither was)
None of them are old enough to drink. (None of them is)
Each of them were incorrigibly greedy. (Each of them was)
The number of graduate degrees he has are very small. (The number is)
The media has altered our perception of politics. (The media have)
The data was inaccurate. (The data were)
The last sentence is yet another example of the evolution of English. Strictly speaking, "data" is the plural of "datum" and thus should take a plural verb. But it's increasingly common to see the grammatically incorrect version used in written and spoken English.