First we need to recognize that "nets" is the subject of "may upset," while "here and there" is a compound adverb. If we were to insert "that" right before may, we would be creating a relative clause to modify the noun before it (which would be "nets"). " "Here and there nets" is not a clause yet.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.