From the thread: 90 ^ 5 combinations, that would take quite a bit of time doing it manually ;-)
90^5 / 3 / 24 / 3600 = approx. 23000 combinations/s
That rate seems much too high, for either a mechanical lock mechanism or an electronic keypad (which I guess would have some kind of lockout for brute forcing anyway).
No idea if that's a common problem with safes, but it's pretty clear that a safe would not have any outside maintenance access to fix a problem like that (other than drilling it open, which seems to be routine enough given the "yeah, we can make it usable again" offer)
2. It's a trap lock and was never designed to be operable in the first place.
Or as others said, the lock mechanism could be faulty or seized.
For example a standard Master dial lock has 40 hash marks but, IIRC, older ones had only about 12 valid positions. There was a trick you could use to find the third position, which would have left 144 combinations try except there were a few invalid combinations reducing to around 120. This is for a lock that a naive estimate would but at 40^3 or 64,000 combinations.
I once witnessed someone open a safe in a few hours. It is truly amazing to see these kind of proffesionals at work. How they approach this puzzle and the knowledge they can acquire by just feeling. Especially considering it took me a few tries to open the safe when I knew the combination.
But its not likely to tell the difference between 50-0-50 and 49-0-49. I suspect the "70% chance" for the robot to succeed is because it's going to try every 4th number or 5th number to hugely reduce the search space.
Trying every 5th number is just 8000 combinations!