User variables seem to have a COERCIBILITY() of 2, the same as table columns, even when assigned values with higher COERCIBILITY. This can lead to unexpected errors:

CREATE TEMPORARY TABLE t (v varchar(100) COLLATE latin1_swedish_ci, w varchar(100) COLLATE latin1_general_ci);

INSERT INTO t (v,w) VALUES ('foo','bar');

SELECT v,w INTO @v,@w FROM t;

SELECT v='foo',v='bar',w='foo',w='bar' FROM t;  # comparison of table columns with literals:  OK because columns have lower COERCIBILITY than literals

SELECT @v='foo',@v='bar',@w='foo',@w='bar';     # comparison of user variables with literals:  OK because user variables have lower COERCIBILITY than literals

SET @f := 'foo', @b := 'bar';

SELECT v=@f,v=@b,w=@f,w=@b FROM t;              # comparison of table columns with user variables; FAILS WITH ERROR:  Illegal mix of collations

SELECT @v=@f,@v=@b,@w=@f,@w=@b;                 # comparison of user variables with each other; FAILS WITH ERROR:  Illegal mix of collations

DROP TEMPORARY TABLE t;

Or perhaps more succinctly, the following look like they ought to be equivalent, but they aren't:

1. SELECT col='value' FROM table;
2. SET @v := 'value'; SELECT col=@v FROM table;

I would like to suggest that they SHOULD BE... and that the way to make this work correctly would be for user variables to retain/remember the COERCIBILITY of whatever value they're assigned.
i.e. in the following:

SET @a := (SELECT column FROM table), @b := 'literal value';
SELECT COERCIBILITY(@a), COERCIBILITY(@b);

Currently the returned coercibility values are (2,2)... but I am suggesting that they should be (2,4) instead, which are the coercibilities of the values that have been assigned to @a and @b respectively.