What happens if I drop a MySQL column without dropping its index first?

According to the MySQL 5.1 Reference Manual:

If columns are dropped from a table,
the columns are also removed from any
index of which they are a part. If all
columns that make up an index are
dropped, the index is dropped as well.
If you use CHANGE or MODIFY to shorten
a column for which an index exists on
the column, and the resulting column
length is less than the index length,
MySQL shortens the index

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