Bootstrap does not support bumping the level

This looks OK. Committed.

This is a pretty serious bug and it exists in D5, too. I've attached a D5 patch that fixes it slightly differently than the D6 patch. I'll adapt the D6 patch if that is what will get committed.

For the record, here is one example (the one I encountered) that triggers buggy behavior:

drupal_bootstrap() can be called more than once including from within other bootstrap steps (e.g. statistics_exit() called during LATE_PAGE_CACHE calls drupal_bootstrap(DRUPAL_BOOTSTRAP_PATH)). As written, drupal_bootstrap() *always* shifts the next phase off the list and performs it.

This is wrong. Consider the scenario where you have two modules, A and B, both of which do what statistics.module does, and the page cache enabled:

function a_exit() { drupal_bootstrap(DRUPAL_BOOTSTRAP_PATH); }
function b_exit() { drupal_bootstrap(DRUPAL_BOOTSTRAP_PATH); }

1. index.php calls bootstrap(FULL).
2. bootstrap(FULL) goes as far as LATE_PAGE_CACHE. At this point, $phases still contains (PATH, FULL).
3. LATE_PAGE_CACHE finds a page cache hit, calls hook_boot(), outputs the cached page, and calls hook_exit().
4. a_exit() calls bootstrap(PATH).
5. bootstrap(PATH) shifts $phases, gets PATH, calls it, and returns.
6. b_exit() calls bootstrap(PATH).
5. bootstrap(PATH) shifts $phases, gets FULL, calls it (generating a "cannot set headers, output already started" error) and returns. Buzzz.

Committed to 5.x.