Just as the title states. Has someone had the misfortune of this occurring?
To this date, I haven't seen anyone losing money from the GUI. When you think about it, it's not that simple: someone has to get not only your password but also the wallet file.
In the sense as is the software secure? Assuming you have a secure password set on your wallet I am unaware of anyone having their coins stolen. It may be that malicious programs could potentially key-log or impersonate your wallet but those are the attack vectors I could see assuming that your password can't be guessed or brute forced.
The GUI itself has not been hacked. The threat models come from what surrounds the GUI such as Operating system, apps installed, and user habits online.
If you are surfing shady sites and clicking lots of stuff, or clicking strange stuff from your emails, chances are your computer has been owned and a keylogger has been installed. Then the attacker just waits for you to enter your password, and he can empty your wallet as he chooses.
If you have a malicious app installed, that may also grab your wallet password.
Windows and Mac operating systems are much more vulnerable to viruses. They also have their own spyware built in which is not normally used for theft, but who knows if an employee in either corporation uses that technology for personal benefit or not.
In conclusion, the GUI cannot be hacked (as far as we know until today), but the rest of the environment has to be considered carefully for high security.