JIRA currently allows you to have post functions in illogical orders that will get people into trouble.

For example, you could update a field in a post function, but have it happen after the history is written and indexing done. This means that the issue will not be searchable using that data in that field, and the edit of the field will never actually show up in the issue history tab. If a plugin (such as GreenHopper) is trying to use this data for calculations it will be invalid - as the field will be in a state that the history does not show how it got there.