One example in networking of a "best practice" that quickly turned into a "worst practice" is disabling auto negotiation. Back in the mid 1990's when 100 mbps

One example in networking of a "best practice" that quickly turned into a "worst practice" is disabling auto negotiation. Back in the mid 1990's when 100 mbps nic's first came out implementation of the new auto negotiation protocol was quite buggy. The workaround at the time was to "lock down" network ports by disabling auto negotiation. Somehow this quickly became canon and an entire generation of network engineers swore this as their first commandment. Meanwhile the bugs with auto negotiation were worked out within a few years and by year 2000 there was no reason to do this anymore and every reason not to: Disabling auto negotiation causes duplex mismatches. If not immediately then eventually it always does. There are too many ways one side of a link can get reset to the industry default (auto negotiation enabled): Replacing hardware, automatic nic driver updates, staff turnover, and simply not checking each and every interface every time a change is made to the network. There is no practical way to keep such an inherently unstable system in sync long term. Ironically these inevitable duplex mismatches only seem to reinforce the belief that you must "lock down" every port everywhere resulting in a vicious cycle of bad practice. The best way to avoid duplex mismatches is to never disable auto negotiation unless you are absolutely forced to by some very old or very poorly designed device.