I used to work at a phone company in Australia. DST was the pits. Think a five minute call where the caller drives across state lines, from a DST-observant state to a non-observant state. Timestamps come from the base station through which the client's phone is connected. several weeks later, client is furious that they have been billed for an hour and five minutes (back when it was expensive).

Every time we switched to or from DST there were a couple of outliers that got caught. We'd update the tables to handle the case correctly next time, but would miss something else, if the change didn't break something else completely.