The culture corona bug is a condition where the boundaries of some captured towns or cities will stay inflated out beyond the normal 9 tile minimum that should occur. The boundaries will collapse backwards down to the 9 tile minimum size some improvement or unit is built to cause the internal program to check and see if this newly produced item will add culture production to the city.
I first reported this bug in June 2002 with a discussion thread in the GOTM8 spoiler threads.
The bug can be a bit annoying and could be exploited to gain a game advantage but many of its advantages can be gained by making certain changes in your game play habits. It is as if this bug actually has taught us to do something that we should have been doing all along.
I'll post more examples and a better description of the bug features here in this document as we can confirm some more independent examples.
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