When not using a prefix with multiple / yet similay projects a mixup of query caching will occur (for example with sfDoctrineGuard queries). This is very easialy fixed with a prefix - but I only realized after two months - that this feature actually existed. There is no mention of this in the documention.
A mention of the feature
$q = Doctrine_Query::create()
Would have helped me and I think it will also help others.