Things to watch for are: A) You signed a contract that explicitly assigns ownership of anything you create to them B) You created it on equipment they own, or

Things to watch for are: A) You signed a contract that explicitly assigns ownership of anything you create to them B) You created it on equipment they own, or used any of their resources to make it C) You created it in time you should've been working for them Generally speaking, the best way to avoid legal problems is not to piss people off too much. So don't build a direct competitor to them and you're probably fine. If you haven't done any of A, B or C, you'd probably win eventually, but could still end up spending money on lawyers just to make it go away.