If a girl is keen on a man, has reached the state of high interaction, and then percieves that he already has a girl, she is likely to withdraw rapidly from him. The interaction will at first go cold. Then she will find an excuse to make a face saving withdrawal from his company.
Such behaviour is a sure sign that she is very interested, and was expecting things to go to a date. Her reaction, in Catastrophe model terms, is as if the man had shown a large avoidance on the high interaction plain - she exhibits a large avoidance, and may take some time to recover and commence her advance again. The more interested she is, the longer the delay may be - perhaps several months.
She is unlikely to be catty on subsequent meetings, as in the Isobel Syndrome, but she will try and avoid any expression of interest - and may take steps to avoid the man. She may also take steps, where relevant and possible, to avoid being seen by the man going out with someone else.
A somewhat similar reaction occurs in the man too. If one has reached a high level of interaction, perhaps near or into Magic, and one suddenly finds the other is committed, then it comes as a stunning blow - what one expected to happen won't happen. There is a feeling of tension, bafflement, disappointment, frustration. There is embarrassment, and a desire to make a face saving withdrawal - and not to give the impression that ones feelings were high.
The reaction will occur even if there is nothing to stop the couple from associating on that occasion, for example if girlfriend/wife is in another part of the country. And naturally this Syndrome is prone to misunderstandings - the girl may perceive incorrectly that the man has a girlfriend etc.; she may confuse his discussion about a former girlfriend with his still having one. Or he may intend to get off with the girl, and accidentally let slip that he has a girlfriend etc.
Yet this Syndrome is more complex than this. Girls who have a record of knowingly pinching other girls' boyfriends will still show this Syndrome. The difference is this: it is the sudden reaction to the unexpected, which will cause disruption, and they can't think properly. But when they are knowingly stalking another girl's man, they know what the situation is, and interpret friendly reaction on the part of the man as making progress and that he might be interested in a change. The discussion in State of Mind provides further background to this - and in fact the emotional state of the girl affects the extent to which this Syndrome occurs. But the strength of this Syndrome is manifest by the fact that it has been observed in girls who in fact have had men elsewhere themselves.
The remedy for this Syndrome is naturally simple - don't give any impression that you already have a steady girlfriend, even accidentally. If one has such a girl and isn't planning to make a change, it is best to indicate this before interaction reaches the high level - in case you might want to go out with her subsequently. She now knows what she's up against - and the chances are good that she will in fact get off with you on that occasion (see also Only to Lose).