Yesterday I lost at least an hour (but only for a coincidence I haven't lose more hours) because I came across a bug of Analysis Services 2000.
If you put a single quote or a double quote inside a comment, the whole MDX expression is not considered. I had the problem in a custom rollup formula, but it's a problem of MDX in any context when the comment is in the middle of a MDX formula; unfortunately, the whole rollup formula is probably considered inside a larger MDX expression so the bug is more critical in this context.
I wrote about the bug in the newsgroup and Deepak Puri kindly answered that it's a known bug, already found by George Spofford almost three years ago (I tried to search about the bug but I haven't found this message before my post). This bug is already known by Microsoft and it's larger than I initially thought: in a later thread (of 2003) it was noted that the bug infect the overall parsing of MDX formula when a comment is inside (and as I said before, in the case of rollup formula the whole MDX formula is subject to this bug).
I hope SP4 will solve this oooold bug, so we don't waste our time next time we use comments in a MDX formula: in italian language the use of single quote is much more frequent than in english...