At the very first job I ever had, with literally hours on the job, my employer somehow put me (at 17, with zero routing experience) in charge of a major change to the primary yet supposedly redundant ISDN routes at a small ISP. Needless to say there were some unhappy customers the next day or two as incorrectly advertised routes drew traffic down the wrong pipe, we figured that out, updated correctly and propagated. Luckily most customers were schools, internet was not yet a critically expected utility, and we buggered things up on a Saturday, so the majority of the customer flak was only on Monday morning. I can't remember what the routing protocol was in those days, but it was probably something early like RIPv1. Meh. Certainly a learning experience I felt bad about, but in hindsight totally not my fault!