On G.I. pistols, made to spec, it's unusual. On a new production gun, made to who-knows-what specs, it's very common. The slide stop should cam the plunger back into the tube, merely by pressing inward on the slide stop, but if the plunger protrudes too far from the tube, either because the tube is too short, or the plunger too long, pressing inward on the slide stop has the effect of trying to bend the plunger. Most people resort to some sort of tool to push the plunger in so the slide stop will pass. Some clever guy has cut a groove in his slide stop, replacing the designed-in camming action on guns that aren't quite to spec.