Having been born and educated in Belgium, the metric system was fed into me from the moment I started school, and although there may have been some non-standard units in use (e.g calories or horse power), metrics such as the metre, litre, kilogram and the watt were already in common use throughout my school and university days.
Metric units were also quite commonly adopted in an engineering environment when I started work in the UK, so apart from the vernacular use of miles, feet, gallons and in the beginning degrees Fahrenheit there was no need to worry of getting your units mixed up. I know that at some point when we were trying to increase our sales into the US there had to be a concerted effort to have their units on any documentation that accompanied the orders, but once the tariff barriers were erected in 2015, that was no longer a major issue.
The only other time I had to deal with US units, and then only in a limited way, was when at Iscor we were attempting to produce pressure vessel steels according to a number of ASTM standards for various steel grades. Even then this remained restricted to the use of psi and ksi (a hybrid unit meaning 1000 psi) for tensile strength properties, and possibly the use of degrees Fahrenheit for specific heat treatment cycles.
The one time that I got fully exposed to the labyrinth that non-metric units can lead you in was during a small part of the syllabus by Prof.Froment, which carried the title of “Stoftransport” (not sure whether the translation of mass transport given on-line really is an accurate one, but it contained things like the speed of a reaction in e.g. a solid pellet when the latter was exposed to a reactive gas which had to diffuse into the solid).
Now fortunately for me the exposure to this environment restricted itself to a one-hour-a-week course during my second year of engineering, but I heard he was a major bugbear to my brother who decided to do chemical engineering, where he was far more exposed to Prof.Froment’s type of teaching. For some reason or another Prof.Froment has his own Wikipedia entry, but in those days he was mainly known for being involved in the US petrochemical industry (in whatever capacity was never made clear to me).
Anyhow, the fall-out from this exposure to the petrochemical industry was that when we had to do any practical exercises, all the examples used a mixture of US and imperial units, and you knew that if you made a mistake in the calculation, it would be 90% sure that this arose from a mix-up in the use of your units such as BTU or gallon, which, despite having the same name represented different quantities depending on whether you were using the US or the imperial variety. Or when you were dealing with weights, was it metric ton, long ton or short ton ? The possibilities for confusion were as numerous as the number of times we fell in the pitfalls of substituting the wrong unit into an equation.
Fortunately none of my mistakes mattered, and subsequent units in my professional life tended to stick with the metric standards as espoused by the various BS or EN standards, so if ever I made a mistake it was one of my own making and not because I was bamboozled by a bewildering variety of similar but different units.