Remeber that DOMNode always needs a reference to a parent node or DOMDocument.

For example, if you try to clone a node - copy all children - and overwrite or delete the variable which holds the cloned node - all children will loose any reference and getting invalid.

This will cause a nice message like "Couldn't... node no longer exists" if you have luck. In most cases PHP only gives you the poor information "Couldn't fetch DOM[...]" which makes hard to find out whats going on, depending on the current operation.