Beware, if there's a backslash present in the key name then the results are unpredictable and random.
I was generating keys like this:
$mc_key = get_called_class(). $_COOKIE['crumb'];
This works fine when get_called_class() returns CLASSNAME.
But then I began using namespaces, and get_class_class() of course returned NAMESPACE\CLASSNAME
Well that backslash sends Memcache into quite the tizzy.
No errors, mind you, just insanity.