Answered by Shaykh Faraz Rabbani
If water from my face goes into my mouth, does this invalidate prayer? Also, if dust/hair accidentally goes into my mouth during prayer does this invalidate the prayer?
Your prayer would be invalidated if you become certain that water entered your mouth AND went down your throat. This is unlikely with a mere drop of water.
With the dust or hair, in order to invalidate the prayer you would have to be certain that it entered your mouth AND that it went down your throat.