Whether you feel the legal assault on file sharers is warranted or not, its foundation and efficacy now appear very shaky. How could such a mistake have been made?

I can think of a number of ways this could happen, some accidental and some malicious. It's easy to transpose a digit. It's easy to get the time-stamp wrong on a request, or foul up the time-zone differences. It's not out of the question that someone at the ISP knew this wasn't valid, but complied out of spite and the letter of the law. And it's also possible that Johnny Thugnextdoor is stealing Grandma Macuser's internet connection because she didn't put a password on her Airport WiFi base station. What we're seeing here is that the DMCA has some serious flaws and that the RIAA will exploit them. It's probably short-sighted on their part, since the lawsuits against 12 year old honor students and Mac-using grandmas will cause a backlash, but isn't one of the long term industry problems the takeover by short-sighted beancounters who can't look past the revenue forecasts?