I'd like to play devil's advocate and attack this thread from a different perspective. I've been an active participant on the Qt forums for many years under the similar handle that I use here so I'm very familiar with the Qt framework. Qt is very complex and it is very easy to misuse its features. In many cases their documentation just isn't adequate and you are left experimenting to figure out how it works. I've seen many so called SO bugs that were in fact triggered by bad Qt API programming: the programmer misused the feature and the framework did something behind the scenes like a double delete of a buffer. I gotta tell ya, this problem has that distinct aroma. Just consider that, and maybe test your hypothesis several different ways. I do understand though that most examples are designed to support a particular view.
I'd really like to know what that Qgl::unmap() is acting upon...segmentation error indicates to me that the buffer has already been released
