A couple of decades ago I witnessed a classic demonstration of Weinberg's Corollary¹ when a spanning tree misconfiguration on a freshly-installed BlackDiamond disabled LINX for hours, throwing half of Britain's ISP peering into chaos. The switch in question was evidently cursed: it'd been dropped on my foot earlier that week, thus disabling me for hours, and everyone else involved in that project was dead within two years.

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[1] "an expert is a person who avoids the small errors while sweeping on to the grand fallacy"