Pouring Milk Isn't as Easy as It Sounds

Imagine the scene, if you will.  It's a normal workday morning.  A colleague from another organisation has arrived for a meeting.  As is the normal social convention, I offer him a drink, and he chooses a coffee, with milk.
I select a mug, pour in a teaspoon of coffee, and put in some hot water, and empty the contents of one of those little buckets of milk into the mug.  It's all given a good stir, but it looks as though it could do with a little more milk.
Not a problem!  We have more UHT pots of milk.
I grab another milk pot, open it up and...
Pour the milk into the bin.
This happens occasionally.  If I have something in my hands, and two activities to undertake, I'll do the wrong thing with the item I'm holding.  I remember some years back holding a used straw, and a stamp (I was collecting stamps at the time).  The straw was to be disposed of, while the stamp when in my album.  And I managed to put the stamp in the coal scuttle.
So, we all make mistakes.  I grab another pot of milk, open it up and...
Yes.  Again.  Poured the milk straight into the bin.
Why?!?