A bone graft can get infected – the mouth is the dirtiest place in the human body and even with our best efforts, infection can set in.
The body could dissolve the bone graft faster than it can heal it, so not quite a failure but it may require us to re-graft it so you have enough.
The body may also flat out reject the bone graft and refuse to dissolve it, in which case we may have to go in and clean it out and at that point we don’t put anything back in so the body can heal the hole how it would normally. Then we may have to try a different grafting technique if there’s insufficient bone to put an implant into.