Many years ago, I was doing a 80 nm cross-country tow with a two-seat Lark (IS-28B2) behind me. We were parallel to and about 10 miles down-wind of the Canadian Rocky Mountain Front Ranges on a wave day.
It was quite rough, so we were using a long rope. About half-way, our CFI, who was flying from the back seat of the Lark, called me up and said incredulously "We're off tow!". Sufficient slack had developed to cause the Tost hook to back-release.
I'll draw a veil of the subsequent field landing and takeoff, other than to say there was no damage, except for grass stains on the belly of the tow-plane - a C182 Lawnmower.
The moral of the story was that we subsequently disabled the back-release, since we only aero-towed.