You're assuming that the fence is right on the border but often that's not the case. In many places Israeli territory extends for a few or even tens of meters beyond the fence. This is often the case where the terrain is difficult. That having been said, I can see how Lebanese forces could've mistakenly beleived that the fence was the border and thus Israeli troops north of the border were on Lebanese territory. Need to wait and see. Hopefully cooler heads will prevail.
UN: Gaza could be 'uninhabitable' by 2020 if trends continue (AP)
from the article: 3 Lebanese soldiers, journalist killed in clash on Israel-Lebanon border