Wudu And Shower
If it happens when one is still in the process of Ghusl then remaining ghusl can make up for that, provided the same requirements of thoroughness and completeness have been applied after that Wuzu nullfier happened.
If one goes to the toilet during Ghusl, then a wuzu nullifier happened, which is something that requires Wuzu again and if one is dong Ghusl then Ghusl again. One can do Wuzu afterwards if one chooses, there is no bar on that.