"Cancel" button in Symbol properties dialogue has the same effect as "OK" button.

[Alister]

1. open the layer properties for a vector layer
2. click the "Change" button on the left hand side of the window. This opens the Symbol properties dialogue
3. change something (e.g. click the + button to add a symbol layer)
4. click "Cancel"
5. click the "Change" button again: you will see that the changes you made last time have been applied (e.g. the new symbol layer is still there), even though you clicked "Cancel".

FWIW I'm running r15740

[Alister]

Sorry: the "Change" button I referred to is in the "style" tab, of course.

(and thanks for fixing my formatting Jef :) )

[jctull]

Also, please note this ticket. ​https://trac.osgeo.org/qgis/ticket/3427

In effect, the ok button causes rendering to occur again, for instance, after already hitting the apply button in the style dialog. The cancel button does not cause this re-rendering to occur, so it does have unintended functionality different from the ok button. At least that is true on my OS X system.