Lets look at this with a bit of logic. Tickets were primarily available to season ticket holders and then later non-season ticket holders too. As I understand it, seating season ticket holders were not guaranteed their season ticket seats once the advanced booking deadline was passed. Therefore we can put the fans that entered the west stand without a standing ticket into three categories:
a) standing season ticket holders who bought a seating ticket but moved
b) seated season ticket holders who bought a seated ticket but moved
or
c) non-season ticket holders who bought seated tickets but moved
Whether it was because they wanted to sample the atmosphere from the different areas on what should have been a special night... because they wanted to stand with friends... or simply because they wanted to stand but were too late to buy a standing ticket only they know, but given the limited capacity of that game I can see why people might not have considered this a problem on the night, nor that it would have such a drastic effect on the capacity status for future matches. Inconsiderate? perhaps. Unsurprising given the circumstances and what we know about people not using their brains to their full extent? definitely not.
Looking at the three categories above I honestly don't believe that it will be an ongoing issue, certainly not something that some better stewarding can't help resolve. I'd like to know the percentages of how many tickets for the game were sold to season ticket holders vs non-season ticket holders. If the vast majority of tickets sold went to season ticket holders then I would suggest that a) and b) will not, or are at least unlikely to happen throughout the season, whilst the possibility of c) happening again is limited due to factors such as
the small numbers of unallocated seats available and could be helped by simply warning fans against moving stands when buying tickets/printed warnings on tickets/via the PA system/signs above the tunnels etc. Short of segregating the areas completely or perhaps doing so after kick off, you'd probably struggle to stop everyone from moving stands but we don't want segregation on the concourse anyway do we.
A bit of common sense is all that is really needed from the fans in future.
I'll also add that the West Stand didn't feel over capacity to me and it was clear that there was more room available towards the North West corner. I imagine better dispersal pre-kick off and at half time by the stewards & fans themselves would have helped in that aspect.