You keep repeating this but the odds of it happening are extremely
remote and if it did occur it would be the result of some awful system
failure / accident / line closure. There are system facilities that can
automatically deal with these situations. If you think about it there
have to be such facilities.

This actually happened to me when Prepay was first introduced. The
clock on the validator at Finsbury Park WAGN was 4 hours fast and I
was charged 2 £5 fees as the system saw 2 unrelated touch ins for a
Kings Cross - Finsbury Park WAGN journey. All sorted out by the
helpline.. :-)