Vitamin K (or Mephyton or phytonadione for you A+ healthcare providers) is the reason why someone on Coumadin should try to keep a consistent diet.  Big changes in intake of vitamin K can cause an INR to fluctuate.  The more vitamin K intake someone has the more INR will go down.  In the event of a significantly high INR, Vitamin K may actually be used therapeutically to try to bring that INR down more quickly than simply stopping the drug by itself would.  Coumadin has a fairly long half-life so in the event of a significantly high INR, it would remain that way for a much longer period of time without Vitamin K.