It's surprising that Berkshire Hathaway Inc. (NYSE:BRK.A) (NYSE:BRK.B) would make this error, because it must run into the regulation relatively often. Basically anytime its holdings in a given company pass $272.8 million (the figure is increased periodically), it has to file. Considering the size of Berkshire Hathaway's portfolio and Buffett's preference for taking large positions in high quality companies and then owning them for a long time, you would expect their compliance team to be on top of this.