The Last (but not the only) NAS FLOP was caused by the well known Irish Gap Problem where a flight ducks in and out of the FIRs.
Don't know who actually tried to do the ammendment on the FPL that caused the FLOP but it doesn't really matter. Since it was / is a known problem it should have been fixed and NAS made unable to accept the input.
BTW ATCOs at Swanwick regularly access and ammend FPLs in NAS.It's part of the Planner Task. Now chant after me " NAS before NERC, NAS before NERC" etc etc etc.
Rgds BEX