I suppose I should have said 'nearly unrecoverable' or 'unrecoverable by standard techniques'.
An example would be Mad Jock's story e.g
Standard Spin Recovery [Archive] - PPRuNe Forums
Or Genghis the Engineer's assertion that many aircraft have been found to have unrecoverable modes: (4)
FLYER Forums ? View topic - Spinning myths - suggestions please?
I don't have the aerodynamic acumen to understand why, and I can't currently find the reference, but I've read a suggestion that aircraft that are designed to be benign in the stall are more likely to have uncommon spin modes that are difficult to recover from.
Then of course there are the 'gotchas' such as having the wrong balance of fuel in the wings e.g. the accident that led to the loss of the ETPS Hunter.