Trigger Old Context Variables (oldMap, old)

Trigger

trigger LogTriggerOld on Account (after insert, after update, after delete, after undelete) 
{
    String BeforeOrAfter = (Trigger.isBefore ? 'Before' : 'After');

    String TriggerType = ''
        + (Trigger.isDelete ? 'Delete' : '')
        + (Trigger.isInsert ? 'Insert' : '')
        + (Trigger.isUpdate ? 'Update' : '')
        + (Trigger.isUnDelete ? 'UnDelete' : '');

    system.debug(BeforeOrAfter + ' ' + TriggerType 
        + ': ' + 'Trigger.old == null    ~ ' + (Trigger.old == null));

    system.debug(BeforeOrAfter + ' ' + TriggerType 
        + ': ' + 'Trigger.oldMap == null ~ ' + (Trigger.old == null));
}

Test

@IsTest
public class TestTriggerOld {
    public static testmethod void InsertUpdateDeleteUndelete() {
        Account a = new Account(Name='Test 1'); insert a;
        a.Name = 'Test 2'; update a;
        delete a;
        undelete a;
    }
}

Log results

USER_DEBUG|[11]|DEBUG|After Insert: Trigger.old == null    ~ true
USER_DEBUG|[14]|DEBUG|After Insert: Trigger.oldMap == null ~ true
USER_DEBUG|[11]|DEBUG|After Update: Trigger.old == null    ~ false
USER_DEBUG|[14]|DEBUG|After Update: Trigger.oldMap == null ~ false
USER_DEBUG|[11]|DEBUG|After Delete: Trigger.old == null    ~ false
USER_DEBUG|[14]|DEBUG|After Delete: Trigger.oldMap == null ~ false
USER_DEBUG|[11]|DEBUG|After UnDelete: Trigger.old == null    ~ true
USER_DEBUG|[14]|DEBUG|After UnDelete: Trigger.oldMap == null ~ true

USER_DEBUG|[11]|DEBUG|Before Insert: Trigger.old == null    ~ true
USER_DEBUG|[14]|DEBUG|Before Insert: Trigger.oldMap == null ~ true
USER_DEBUG|[11]|DEBUG|Before Update: Trigger.old == null    ~ false
USER_DEBUG|[14]|DEBUG|Before Update: Trigger.oldMap == null ~ false
USER_DEBUG|[11]|DEBUG|Before Delete: Trigger.old == null    ~ false
USER_DEBUG|[14]|DEBUG|Before Delete: Trigger.oldMap == null ~ false

Trigger.Old is null in a Before Trigger and will therefore throw a NullPointerException for any reference / method invocation on it.

16:08:40.060 (60355000)|USER_DEBUG|[2]|DEBUG|**** Trigger.old null 16:08:40.060 (60362000)|SYSTEM_METHOD_EXIT|[2]|System.debug(ANY) 16:08:40.060 (60503000)|FATAL_ERROR|System.NullPointerException: Attempt to de-reference a null object