I'm having trouble finding a rule.

[Hyozo]

What are the rules for when, for some reason other than plyer oversight, a character gains a feat which he doesn't meet the prerequisites for. As an example, a first level cleric with war domain has a 0 BAB, but war domain gives weapon focus, which requires a BAB of +1 or higher.

[JackMage666]

If it's a bonus feat like that, which he doesn't get options, it still works. Generally, it will say "even if he doesn't meet the pre-requisites" in the description, but it's not always the case.

He cannot, however, take Improved Critical as a 1st level Fighter Bonus Feat, because he doesn't meet the pre-reqs. But this is given to him, so it's fine.

[Shhalahr Windrider]

As JackMage said, Bonus Feats that don't grant you a choice generally apply whether or not you meet the prerequisites. A good example of this is the Monster Manual errata. There were a number of creatures that had originally been granted Weapon Finesse as a normal feat but lacked the +1 BAB prerequisite. The errata changed Weapon Finesse to a bonus feat and gave them a replacement normal feat (usually Alertness or Stealthy).

Though, it should be noted that in most cases, this is explicitly called out in a class description. The War Domain granted power does not do so, however.

If the player has a normal feat for which he or she normally qualifies, but then the character loses access to the feat until such a time he or she regains the prerequisite.

[Hyozo]

Okay. That is good to know. Thank you.