undefined behavior (UB) is the result of executing a program whose behavior is prescribed to be unpredictable, in the language specification to which the computer code adheres

Examples:

• Division By Zero
• Memory accesses outside of array bounds (out of bound access)
• Signed integer overflow
• Null pointer dereference
• Modification of string literal
• Accessing a NULL Pointer, etc.

References

• https://en.wikipedia.org/wiki/Undefined_behavior
• https://www.geeksforgeeks.org/undefined-behavior-c-cpp/