System Information
ubuntu 14.04 64 bits Nvidia Titan Black

Blender Version
Broken: 2.74.5 hash 1a0fb7e
Worked: (optional)

Short description of error
In compositing nodes, when blend type of mix node is changed with alt key pressed, options of selected nodes (that have nothing in common) can change.

Exact steps for others to reproduce the error
Based on a (as simple as possible) attached .blend file with minimum amount of steps

In this file, change blend type of mix node while pressing alt Key from Mix to Add or Substract or Divide....
By this action variable size option of blur node is enabled.
Change it to Mix or Multiply or Screen ...
Variable size option is disabled.

Same kind of things can happen with mask type of a Mask Type node.