Cup holders break easily, especially if sticky liquids are spilled into the cup holder mechanism.Google+
Car Problem Reports
Mercedes-Benz E320 Spilling Sticky Liquids Around Cup Holder May Cause Lid to Stick/Break
Mercedes-Benz E320 Problem
Average Mileage: 132,000 mi (80,000 mi - 176,000 mi)
Visitor, 2000 Mercedes-Benz E320, 80,000 mi
replaced cup holder.
That is what happened spilled liquids around and down cupholder and it stick and broke.