In a campus system, it was observed that a number of networked PCs crashed occasionally, but only late on Friday afternoons. The only known change to their software was the introduction of a recently written Ethernet device driver. The eventual bug was found to be that a periodic broadcast packet on the network advertising the number of users on another particular machine, exceeded the buffer size for received packets in the PC. Normally, PCs only talked to each other, and therefore never sent large packets, or large broadcast packets. Most of the week the large machine had fewer users. But on a Friday, coursework was due in, so many more students used the machine. In software terms, the problem was that the check on the packet length was after the check on who the packet was for (and subsequent copy of the packet into an insufficiently large buffer).