One thing editors look for is subject/verb agreement, basically, if the subject is plural, the verb should be too. This gets more complicated with long subject phrases where there are several nouns. Here's an example of one that kind of stumped me for a moment. Usually I can "hear" the right verb, but this one... well. Took a moment's thought:
He pointed to the 2008 public offering, which exemplified the threat Facebook and the other social media companies pose(s) to Microsoft and other traditional internet companies.
Hmm. What poses? The threat? (singular, so "poses"--ever notice how "s" signifies plural in nouns, but singular in verbs?) Or "Facebook and the other social media companies?"
The latter. But I had to parse that, because the "ear" first put "threat" as the subject. Fortunately, the other "ear" is more skeptical, and "heard" something off there. So lesson is: When a sentence is complicated, read more carefully.