I see it allll the time working at a fish shop. Another common defect is crooked spines on tetras and danios. I would put it back in with the rest since tetras do best in schools, and keep an eye on it.
And right now there is a tiger barb with the "missing mouth" but it does just fine. It seems like it just inhales food rather than having to grab it with a mouth, so its the fattest of the bunch.