What is the value of the number field of the object that is obtained from Trigger.old?
The initial value for a number field on a record is 1. A user updated the value of the number field to 10. This action invokes a workflow field update, which changes the value of the number field to 11. After the workflow field update, an update trigger fires.¬†
Trigger.old contains a version of the objects before the specific update that fired the trigger.
However, there is an exception. When a record is updated and subsequently triggers a workflow rule field update, Trigger.old in the last update trigger won’t contain the version of the object immediately prior to the workflow update, but the object before the initial update was made.