$ tex --version
MiKTeX-TeX 2.9.3759 (3.1415926) (MiKTeX 2.9)
With the attached file, run:
$ makeindex crash.idx
And makeindex will crash. I get a dialog saying "General Purpose Index Processor has stopped working" in my locale.
The file crash.idx has two index entries in it. If I remove the first entry, the remaining entry is rejected:
!! Input index error (file = refman.idx, line = 1):
-- First argument too long (max 1024).
As far as I can tell, the problem is that the last character in the argument, when truncated to 1024 characters, is a backslash. Other characters at that position do not trigger the crash.
I don't know why it matters that this index entry is not first. Any other position in the list of index entries will trigger the crash.