I had one a few years back which highlighted issues with both our attention to the network behavior, and the ISP's procedures. One day the network engineer came over and asked if I knew why all the traffic on our upstream seemed to be going over the 'B' link, where it would typically head over the 'A' link to the same provider. The equipment was symmetrical and there was no performance impact, it was just odd because A was the preferred link. We looked back over the throughput graphs and saw that the change had occurred abruptly several days ago. We then inspected the A link and found it down. Our equipment seemed fine, though, so we got in touch with the outfit that was both colo provider and ISP.
After the usual confusion it was finally determined that one of the ISP's staff had "noticed a cable not quite seated" while working on the data center floor. He had apparently followed a "standard procedure" to remove and clean the cable before plugging it back in. It was a fiber cable and he managed to plug it back in wrong (transposed connectors on a fiber cable). Not only was the notion of cleaning the cable end bizarre -- what, wipe it on his t-shirt? -- and never fully explained, but there was no followup check to find out what that cable was for and whether it still worked. It didn't, for nearly a week. That highlighted that we were missing checks on the individual links to the ISP and needed those in addition to checks for upstream connectivity. We fixed those promptly.
Best part was that our CTO had, in a former misguided life, been a lawyer and had been largely responsible for drafting the hosting contract. As such, the sliding scale of penalties for outages went up to one-month free for multi-day incidents. The special kicker was that the credit applied to "the facility in which the outage occurred", rather than just to the directly effected items. Less power (not included in the penalty) the ISP ended up crediting us over $70K for that mistake. I have no idea if they train their DC staff better these days about well-meaning interference with random bits of equipment.