Indexes are absolutely key in Filemaker, and a corrupt index may give wrong results, and hence give users false data.
But, unfortunately they can fail very silently. Yesterday a ExecuteSQL query failed, and after some head scratching, I discovered that the index was bad. I was lucky to notice it, it would have cause wrong data presented to my users.
So i wish there was some kind of mechanism that would constantly monitor and detect bad index. Maybe a builtin checksum ? Or at least a process in FMS (like the integrity checking) that would close the files, analyze their index, rebuilt them if necessary.
Nowadays, unfortunately, one can't assume his solution indexes are perfect. That makes the filemaker database unreliable.