Currently, if you create a windowed SFML application in Windows (XP - haven't tried Vista), and then drag the window partway off of and then back onto the screen, you get a 'smearing' visual artifact due to the window not updating until the mouse button is released.
Most of the other cross-platform libraries I've tried (SDL, GLFW, etc.) have the same behavior. I've looked into the issue a bit, and unfortunately, it doesn't look like it's entirely trivial to solve, and I imagine fixing this behavior in SFML would probably require significant restructuring of the windowing and events system.
Nevertheless, I thought I'd throw it out there. I've been wrestling with this issue (my project currently uses SDL), and will probably end up re-writing the app to use the native Windows API in order to work around it. In some contexts this 'smearing' probably isn't that big a deal, but for other applications, having the window continue to render correctly as it's moved around the screen can be important.
(This issue may already be in the roadmap, but if so I didn't spot it. Also, if there's a way to work around this problem in SFML, I'd be interested to know about it.)