Dangling Modifiers
I love the name of this mistake -- it makes me think of a dramatic, life-or-death situation such as hanging precariously off a cliff. (Of course grammar mistakes are never that drastic, but it helps me remember to keep them out of my writing.)
This mistake happens when a descriptive phrase doesn't apply to the noun that immediately follows it. It's easier to see in an example
After declining for months, Jean tried a new tactic to increase ROI.
What exactly is declining for months? Jean? In reality, the sentence was trying to say that the ROI was declining -- not Jean. To fix this problem, try flipping around the sentence structure (though beware of passive voice).
Jean tried a new tactic to increase ROI after it had been declining for months.
Better, right?
