Cases where Linux can be made to Crash in CPL3 (User Mode) without Root Acces?

Respected Seniors,

While the Current Privilege Level CPL is 3 (which means that the system was in user mode) if we encounter a Crash that normally means that the hardware is faulty as System should not crash in User Mode. However there are some cases in which system can be made to crash without

SOME KNOWN METHODS,

Fork bomb, intentionally causing Memory leaks

Operating System Considering: Ubuntu

Question:

What Other possible scenarios can cause such crash without hardware failure. Please also give some Code references for the Understanding.