The problem was that it was possible to move the TDB (Track Data Block which gives callsign, level, ground speed and other info) around relative to the target symbol and, provided that the TDB remained close to the target symbol, there would be NO leader line (or stick, or pointer or whatever you want to call it) connecting it with the symbol. (There are normally no leader lines anyway.) The line only appeared if you moved the TDB further away.
Most people get used to having the TDBs in a certain position relative to the target, and will automatically know which one goes with which. Once you start moving them about then you need to think about which is which. This can cause confusion, as in this case. I seem to remember (but I may be wrong) that in the incident that was referred to the aircraft had actually crossed flightpaths subsequent to the controller having moved the TDBs (whilst vertically separated) which made the situation doubly confusing, as it was quite feasible for the TDB in one position to be connected to the wrong aircraft.
As I understand it, the new software now makes it impossible to move the TDB even the smallest distance from its default position without generating a leader line back to the target symbol. You might well ask why this wasn't in from the beginning... but there we go.
