As you say Skiiltan, being in 6th form, they are still in most cases at that stage of learning about these things, so errors will be made by some.
I expect that's one of the reasons many schools expect applicants to have work/volunteering experience because that will be part of the parcel. DD wrote her first official letter to give notice to her week-end job two weeks ago (to focus on A levels), it was very new to her. Some applicants won't have that experience.
And even with that experience, just like us adults, errors are made even with important matters, appointments missed, passports left behind before going on holidays...
As you say, applicants can end up being penalised for it, thankfully in the case here, Lonicera's DD realised just on time fort he problem to be fixed.
I think we've all had the worry that our DC received an invitation to interview that has gone to their junk mail and got lost as a result!
Good luck Somerset with Sheffield tomorrow.