Ok, I need help.

In several manuals, including a Ford Owner's manual (forgive me) and a
Haynes repair manual, I've noticed that they specify that the negative
battery cable must be removed first when disconnecting the battery.  Why???

What possible difference can it make which terminal is disconnected first???
 When I first ran across this, I ignored it as a harmless error by someone
who didn't understand electrics, but now I find it in "professional"
references.

Help!  What am I missing?

//Jack