You most probably still have the disk partitioned in the GPT, but there are no partitions on the disk any more. This has happened to me twice when trying to remove the Boot Camp partition.
You have to compare the output of
diskutil list disk0 and
sudo gpt show disk0 to see which partitions are still in the GPT, but are marked as unused. Then you'll have to create a temporary partition which you can then delete from Disk Utility.
[onik@Oniks-MacBook-Air ~]$ diskutil list disk0
/dev/disk0 (internal, physical):
#: TYPE NAME SIZE IDENTIFIER
0: GUID_partition_scheme *251.0 GB disk0
1: EFI EFI 209.7 MB disk0s1
2: Apple_HFS Macintosh HD 204.0 GB disk0s2
3: Microsoft Reserved 16.8 MB disk0s3
4: Apple_Boot Recovery HD 650.1 MB disk0s4
onik@Oniks-MacBook-Air ~]$ sudo gpt show disk0
start size index contents
0 1 PMBR
1 1 Pri GPT header
2 32 Pri GPT table
40 409600 1 GPT part - C12A7328-F81F-11D2-BA4B-00A0C93EC93B
409640 398437504 2 GPT part - 48465300-0000-11AA-AA11-00306543ECAC
398848000 32768 3 GPT part - E3C9E316-0B5C-4DB8-817D-F92DF00215AE
399109288 1269760 4 GPT part - 426F6F74-0000-11AA-AA11-00306543ECAC
490234719 32 Sec GPT table
490234751 1 Sec GPT header
shows that there is unused but reserved space at the end of the disk (the line after index 4). You can solve this by creating a partition in the space manually.
Boot into recovery and open a terminal. Unmount the disk with
diskutil unmountDisk disk0 (use your own disk number) and calculate the available space for the partition. OSX needs 262144 sectors of empty space after each partition, so subtract that from the size of the space available. After this, create the partition with
sudo gpt add -b 400379048 -s 89829462 -t hfs disk0.
After this, if you run
diskutil list disk0 again, you'll see the partition, but you'll need to format it.
sudo newfs_hfs -v "Temporary volume" -J /dev/rdisk0s5 will do the trick (but use your own disk and partition number). Now you can close the terminal, open up Disk Utility and delete the temporary partition, and Disk Utility will take care of the rest.
Now just reboot and run Boot Camp Assistant again!