Proper English language usage
The verbs "set" or "step" and the noun "foot":
Distance - "Don't you dare step one foot over my threshold!"
Here, the 'one foot' is a measure of distance; twelve inches. It has
nothing to do with the appendage at the bottom of your leg.
Action - "Don't you dare set one foot on my clean floor!"
Here, the 'foot' implies one (or both) of the the appendages at
the end of your leg which are clad in muddy work boots. It doesn't
matter how far in the muddy boot prints would go, just don't get
your muddy boots on this floor at all.
"Step into my parlor..."
Definitely a direction and/or distance.
"As soon as he set foot on the first flagstone, the trap was set."
Again his foot was placed on something, the
poison-tipped darts flew.
"It was a misstep, which she regretted immediately."
This one is probably allegorical, isn't it?
-- Absolutely dead wrong: --
"Don't you dare step foot over my threshold!"
Here, the 'step' is a verb with an unequivocally inappropriate
and stupidly misapplied object ('foot'). This cringeworthy abuse
of the language pops up with monotonous regularity on television and
radio. It makes one wish there were a button on one's TV remote which
would apply a good slap to the back of the on-air personality's head.
Repaired - "Don't you dare step over my threshold!"
Here, the 'step' is a verb which already implies using either or both feet.
The distance is not twelve inches, not even an inch, not even a nanometer!
Please, please use this phrase instead. Thank you.