The yellow jersey team would never let a competitor get into the breakaway period, they would just ride his wheel. Even in the late 2000's when the tour was decided for 3 years with less than a minute lead, no one challenged the yellow. The only way I could see it ever happening is if a massive cross wind would be around on the final stage before Paris and split the peloton into two and the yellow get caught in the 2nd peloton. I have to look but I think LeMond won by 8 seconds, I wonder if his closest rival (who I think was a team mate of his), did any challenges on the Paris stage.