To reproduce:
1. Import the attached animator. It has a few Blend trees that can be used for testing this
2. Open the animator window
3. Double click locomotion
4. Double click run - you should now have a blend tree selected. Notice the threshold fields in the inspector
5. Left click on any threshold field (on the right side of the numbers) and while holding the mouse down move to the left of the numbers. Sometimes this works as intended (part of the number get's highlighted) but other times the bug occurs:

The whole number remains highlighted, but the white marker moves along with the mouse. If a number was selected at the start of the click - the number replaces the one at the marker when the mouse is released (which is a pain, since it's hard to notice that something got changed).

This might take a few tries, if it doesn't happen on one threshold field, try another. Note that this only happens on the first select, selecting the same field again will work fine.