Two quick theory questions

[bwv772]

I have just a few quick theory questions, I'm trying to understand what I'm reading.

First, what's an unprepared dissonance?

My second question is this: I'm reading "The Patterns of Invention" by Dreyfuss where he analyzes Bach's first invention. The author discusses measures 7 and 8 and describes what would happen if they were strictly inverted from measures 1 and 2: The measure 7 7 RH would go G GABCABG and D D DEF#GEF#D for measure 8.

The reason there's an F# instead of a D in measure 8 is that "An unprepared dissonant fourth occurs at the beginning [of this inversion] while the progression at the end of the measure, though correct in terms of strict counterpoint, had lead to a misleading false accent on a root-position chord which falls misleadingly on a weak beat."

I keep re-reading this trying to understand but I can't! What is meant by "the progression had lead to a misleading false accent on a root-position chord which falls misleadingly on a weak beat."

Many many thanks for your replies.