[tisme]

It looks like someone forgot the locktite. The rover must have experienced lots of vibration on its long voyage, anything that wasn't secured properly could have vibrated loose.

Vibration is a pretty weird thing, the forces can be enormous with very little visible effect until something suddenly breaks or pops loose.

[weinzierl]

When I worked in satellite design we never used loctite because of outgassing. Washer thickness and diameter as well as torque was calculated for every screw and bolt. At the assembly at least two people were involved, one would use a digital precision torque wrench to apply the calculated nominal torque, another one would document the applied torque besides part number, date, time, etc. in a report. Assembly in aerospace is a bureaucratic process.