Call to undefined function simplepie_require()

With Simplepie.module installed, errors may occur when trying to invoke Simplepie

Fatal error: Call to undefined function simplepie_require() in sites/all/modules/activitystream/activitystream_feed/activitystream_feed.module on line 13

This is due to module ordering. If Activity Stream runs before Simplepie, the call will fail. If it runs after Simplepie, no errors occur.

Updating to the latest -dev release (remember to run update.php after any module update) will solve this. You may also just run the DB update that the latest -dev release adds:

UPDATE {system} SET weight = 100 WHERE name  = 'activitystream';