I can't imagine what would have caused that. I have been using S23 since it was very first introduced in '98 or '99. Prior to that I used WYeast 2308. I made the switch due to the ease of use in a commercial setting. Since brewing schedules in a pub are directly dependent upon over the bar business, it can sometimes be a little difficult knowing EXACTLY when a tank will be empty, so ordering massive amounts of yeast from WYeast or White Labs could be difficult.
ANYWAY, I have used this yeast both at home and in a pub, fermenting it between 35F and as high at times as 65F (when ambient temp in the house got too high), I have had ZERO issue with any kind of off flavors. Resulting beers have been every bit as clean as 2308... slightly different palate, but every bit as good. I found 2308 to give a soft palate with a slight, pleasant yeastiness to it, not intense at all, but present... S23 gives me a very round maltiness. I have made these styles with S23; Czech Pilsner, Oktoberfest/Marzen, Schwarzbier, Doppelbock (maybe one or two others). Never once an issue with off flavors. So, my question is, what are people doing to make the off flavors happen?