Common Runtime Exceptions and What They Really Mean

System.NullPointerException: Attempt to de-reference a null object

Class.OpportunityTriggerHandler.afterUpdate: line 12, column 1

This message pattern — "Attempt to de-reference a null object" — always means code tried to call a method or access a field on something that was null. The stack trace beneath it (line 12) tells you exactly which line, and from there, the fix is almost always Module 12's null-check guard, added at that specific point.

System.ListException: List index out of bounds: 5

"List index out of bounds: 5" means code accessed index 5 on a list that doesn't have that many elements — the number in the message is the index that was attempted, immediately narrowing the search to wherever that specific index came from (often an off-by-one mistake, Module 7's classic-for-loop trap).

System.DmlException: Update failed. First exception on row 0;
first error: FIELD_CUSTOM_VALIDATION_EXCEPTION,
An Opportunity cannot be marked Closed Won without a positive Amount.: []

A DmlException's message usually names the actual underlying reason — here, FIELD_CUSTOM_VALIDATION_EXCEPTION plus the exact custom message from Module 26's addError() call. Reading past the generic "Update failed" prefix to the specific reason underneath it is the key skill: the real cause is almost always spelled out directly in the message, not something to guess at.