the only real danger to casting in public is their reaction to you preforming it. there won't be any lightning falling from the sky, and depending on the spell you're casting i don't think many people would know you were casting. if they did find out, depending on the area, it would probably result in you being attacked [verbally or physically] by people who think you're a 'devil worshiper'.
No danger at all, it would just be pointless, like everyone else is saying, it depends on what spell. The real danger is people thinking you are crazy for doing a spell out loud in public. Other than that you will be fine and also might need to learn more about magik.