Blender is automatically selecting multiple vertices while not holding down shift, help? So I am just working on a chair in blender and opened it up and I immediately ran into this problem.
I would select a corner of a cube and move it into a position, done with it, and then go to select the next corner, which should automatically deselect the previous corner and only select the current corner, except Blender just isn't cooperating today and selecting both corners as if I was holding down shift to select them both. I have obviously pressed a button or something which makes it do this but I couldn't find anything when I searched it up so I need your guy's help. Thanks guys.
Images:

Nothing selected here

Box select first corner

Box select second corner, not holding down shift
At the top of the window you should see the select tool 'Mode' this indicates whether it should Replace, Extend, Subtract, Invert or Intersect the selection - presumably you have somehow changed the mode to Extend (add to the existing selection).

Note that as well as the selection tool in the tool shelf you can alternatively press B for a box select. This does not use the same selection mode and instead defaults to 'Extend' the selection - unless you hold Shift to indicate Subtract.