Here’s one that’s not as consequential as other posts here. It’s not going to change the world, but would make things slightly better.

Split lock washers are worse than useless. They’re supposed to be a spring against the bolt to help resist it turning back out over time. They don’t. If anything, they make it worse.

Here’s a NASA publication on fastener design (because of course there’s a NASA publication on fastener design): https://ntrs.nasa.gov/citations/19900009424

The lockwasher serves as a spring while the bolt is being tightened. However, the washer is normally flat by the time the bolt is fully torqued. At this time it is equivalent to a solid flat washer, and its locking ability is nonexistent. In summary, a Iockwasher of this type is useless for locking.

This was published in 1990, but we’re still using this shit. Stop. There are many other kinds of fastener locking that work, like nylon locking nuts or threadlock, and we don’t need these.