One of my coworkers years ago, a Rubik's fan, got a job once in small part because he solved a cube in the boss's office and pointed out that someone was fucking with him by rotating one of the edges 180º, making it unsolvable.
Did you mean flip an edge, or 120°?
If you got to the point where you can routinely solve cubes, i.e. you know your algorithms or you developed a good intuition, then unsolvable cubes should be pretty obvious. Some algorithms are like: if you are in this position, do this, if you are in that position, do that, other positions are illegal. If you are in an illegal position, then you know there is a problem with your cube.
The Rubik's cube is a really hard puzzle unless you know some theory, and the existence of unsolvable cubes is a rather well known fact, meaning that there shouldn't be that many people who are able to solve the cube and don't know about unsolvable cubes.
Or maybe that's just me. I learned on a terrible cube that had a tendency to dismantle itself, so knowing that it had to be put back properly was a requirement for a successful solve.