NavList:

 

Perhaps one of the greatest disadvantages to the introduction of new systems is that they rarely effectively or entirely supplant the old.  More typically use of the new gets added to use of the old almost indefinitely into the future, to the confusion and thus frustration of many ... as yet unborn generations.  

 

We've had comment on this already (nuts & bolts, etc).  Sea-going navigators have, most likely, learned to check charts almost automatically to see whether depths or fathoms are indicated.

 

Another good example may be the multiple scripts currently used for written Japanese, combining Chinese characters (the first writing system adopted) and two (or more?) subsequent purpose-designed scripts.  In practice none of the reformed scripts has entirely replaced older forms, although some books (and especially comics!) etc, are entirely written in the more modern kanji.  Nevertheless, it is common to see a mish-mash of Chinese and other characters in written Japanese.