SQL Server 2008 R2 index rebuild fails with severity 17
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.