MySQL error: “Column 'columnname' cannot be part of FULLTEXT index”

Question

Recently I changed a bunch of columns to utf8_general_ci (the default UTF-8 collation) but when attempting to change a particular column, I received the MySQL error:

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Answer 1

Oops, so I have found the answer to my problem:

All columns of a FULLTEXT index must have not only the same character set but also the same collation.

My FULLTEXT index had utf8_unicode_ci on one of its columns, and utf8_general_ci on its other columns.