I was evaluating the performance of a home's foundation today where a root barrier had been improperly installed to low in the soil.  This resulted in the barrier being pushed down as an oak tree's roots grew over the barrier.  The failure of the barrier was evident in the performance of the building's foundation which had not shown much evidences of differential settlement over the last 15 years, and which had shown significant changes in performance over the last few months.

The root barrier is the sliver line the tree root is to the right in the picture.

Root barriers should be installed well above the soil to prevent this from occuring.  If you are not installing them above grade then they will eventually fail.