sometimes it happens if their servers doesn't respond to queries.
check the who is info for it and see when was it registered.
it happened to me aswell with register.com. it showed creditcards.ws as available, i paid for it and the registration came back as failure. that domain was registered from past few years when i checked the who is record.
GoDaddy is known for showing .ws names as being avail. when in fact they arnt. If the registry is down it will show the name as being avail., always check the name elsewhere, I use NameCheap.com to verify if the name is truly avail., if the registry is down it shows an error rather then saying the name is avail. to reg.