Today I had to recalibrate my monitor again as the last calibration had been some time ago. Since I own a Spyder 4 calibration device I always had some trouble with inconsistencies between different calibration runs and this was also the case today. One time the calibration resulted in a green tint, the next time a bit less and another time even in a bit of a red tint. After googling around I found a possible candidate for explaining this issue: ambient light.
Of course I followed the advice to avoid strong light falling directly on the screen, but it seems that even the remaining ambient light in the room from a windows with closed curtains is too much, at least for the Spyder 4. So what I did was putting a thick black cloth on the screen (and the Spyder device) during the calibration run to shield it from all ambient light. And voilà, results are much more reasonable and consistent.
So, don’t underestimate the effect of ambient light on the monitor calibration. This seems to be especially true for glossy display types. In case of doubt, use a dark room or put a thick black cloth on the monitor during calibration.