SQL Server 2008 R2 index rebuild fails with severity 17 Occasionally during our index maintenance, the job will fail with a SEV 17 error where enough space can not be allocated for the object it is rebuilding. The database is laid out as such:

Data_file1    PRIMARY    0 growth         0% free                Max Size UNLIMITED
Data_file2    PRIMARY    0 growth         0% free                Max Size UNLIMITED
Data_file3    PRIMARY    0 growth         Less than 1% free      Max Size UNLIMITED
Data_file4    PRIMARY    250 MB growth    Less than 1% free      Max Size UNLIMITED


Essentially, 3 of the 4 data files are full and not allowed to grow, the fourth is full and allowed to grow. The files are spread out across different LUNs (and the reason for why is messy). So when the online index rebuild starts, it is my understanding that if any additional space is needed, it will grow into Data_file4 and be fine, but it is apparently trying to grow into a different file where growth is not allowed and failing. I am unable to reproduce this error, but I was wondering if anyone had insight into why this happens.

Full SQL Server version is 2008 R2 Enterprise, SP2 CU 4 (10.50.4270). We use Ola Hallengren's rebuild scripts, where we rebuild online but no sort in tempdb.
My experience is that it is always going to do an online rebuild in the filegroup on which the index lives.  It has to map the existing index and hold enough space for, essentially, one copy.

You should only be getting the error when an index which is too large to hold mappings (the copy) is rebuilt - for instance, one time it may be fragmented enough to qualify in Ola's script and the next time it may not be.

There is a great article http://technet.microsoft.com/en-us/library/ms179542(v=sql.105).aspx  which I had to read several times when running into disk space issues with indexes.