Having both in use simultaneously like we have in Canada, where we work with lots of things manufactured in the US or in Canada for export to the US.

So any technician will have to have both metric and imperial tools, occasionally things of very similar size will get interchanged accidentally - using a 3/4" socket on a 19mm bolt for instance, which will work for a while but eventually round off the head because 3/4" is slightly more than 19mm.