We have, as the human society, have made some mistakes that have become so entrenched that there is no hope to correct them.  But it is amusing to think about them and imaging a world had the mistakes not been made.

Traffic Signals

A real big mistake was made when traffic lights were invented.  Red of course means to stop, Yellow means caution, and Green means to go.  Then someone got the bright idea of Red Stop Signs.  First of all, Red is the most irritating colour.  If you don't believe it, wave it in front of a bull.  Then Green is a pleasant colour.  Now, the question is, why didn't we come up with a system that didn't use red?  Think of the advantages.  If we had "Go" lights we would, in most cases, need only one green light instead of the traditional three red, yellow, green lights.  We would save power, only one light per intersection.  Sure the rules would change. If the light was not lit, the driver must stop at the intersection.  Ah Ha, if the light was burned out, or the power was out, the danger would be removed because everyone would stop before entering the intersection.  Now about "Stop" signs.  If we converted to "Go" signs we would not have accidents with signs that were pushed down or were obscured by vegetation.  And all this in addition to drivers being treated to being able to continue through an intersection with a pleasant green sign instead of punished by a red stop sign.

In the world of physics and electricity and electronics:

Some how the poles of an electric battery have been misnamed.  Anyone who has had to deal with batteries know that there is a positive + pole and a negative - pole.  Interestingly, the positive pole has a diffenciancy of electrons and the negative pole has an excess.   Therefore when the two are connected by a conductor, electrons flow from the negative pole to the positive. Current, for many years considered to flow from the positive pole to the negative.  To me, that is like defining water flow as being up-hill.