contextual flags with OpenGL 3.0 and above are currently messed up. Really doesn't have that big of an effect though(i.e. can't force a core profile and can't use a debug context).
EDIT: Also stated but something I should make sure you don't do... don't mix SFML rendering functions with other rendering functions. You end up with messed up OpenGL state (which can be seen with tools like apitrace) that will just confuse the absolute hell out of you.