The first one is usually caused by people who repeatedly engage park with the vehicle moving, or who park the vehicle on very steep inclines and don't use the hand brake as well as putting the car in park. The pawl that 'locks' the transmission output shaft is only designed to offer secondary safety in addition to applying the park brake - not be a substitute. Repairing it requires quite a bit of work as it is inside the transmission.
In the second the battery is either on the way out, the starter lockout relay isn't being operated by the transmission selector position switch properly (switch out of adjustment or contacts dirty/corroded), or the relay itself is suspect. All pretty easy fixes.
The first is a common issue on any automatic transmission that has been abused. The second happens as the cars get a bit older but is easy to fix, so I wouldn't exactly call it an 'Achilles heel'.
'94 XJ12 (and other toys)