Data Aborts are generated when a user's BASIC program accesses non-existant memory.  One way is accessing an array with an index that is larger than available RAM space.  Another is using a pointer for hardware access, but with a value that does not correspond to a valid location.

Prefetch aborts indicate an attempt to access an instruction from non-existant memory.  Prefetch aborts can occur when RETURNing when a sub/function had not been called.

Undefined Routine, which indicates a call or return to non-existant code.  This error will occur if you RETURN when there has not been a GOSUB, the equivalent of a return stack underflow.  This also may occur when interrupts are used with firmware prior to version 7.30

The number reported (in hex) is the program address where the illegal access was detected.