After hearing reports from several Web administrators who encountered problems with Microsoft Index Server 2.0 after applying the Malformed Hit-Highlighting Argument hotfix, I conducted a brief test on my own Index Server. I applied the hotfix, rebooted a few times to see that the service started, initiated a rescan, and made a dozen or so queries. I didn't encounter any problems.
Don't confuse this testing with the thorough testing that you should conduct before moving an application to a production server. Microsoft doesn't conduct regression testing on hotfixes like it does on service packs. Your setup might have a particular combination of elements that could result in problems when you apply a hotfix.