Airports like Stansted which use stand entry guidance systems have a procedure where the ground handling staff switch on the guidance when the stand is clear and in readiness for an arriving aircraft. If the entry guidance is not switched on the pilot should not park on the stand as it could be unsafe. Seems like in this case the entry guidance was switched on both stands LHS and RHS for the Ryanair and Easyjet and one started to park on the wrong side. Aircraft following one another towards similar parking areas should be aware of each others movements and apply sufficient separation to ensure that in events such as this there is ample time and distance to deal appropriately with the situation safely. "Slammimg" on the brakes and narrowly avoiding hitting the other aircraft sounds a bit extreme.