System Information
Operating system and graphics card

Blender Version
Broken: Blender 2.70, last version
Worked: (optional)

If you are in orthodraphic len, and in a fron, left or other views with the num pad (1,3,7), and try rotate wuth de midle button of the mouse, the orthographic len is losed and turn on de perpective len.

Exact steps for others to reproduce the error
Put the len in orthographic. press one views (1,3,7...), whatever you want. Rotate with midle buton of mouse. Show that the orthographic len has losed