NISAM.
ISAM when doing some ORDER BY ... DESC queries.
--delay-key-write didn't enable delayed key writing.
TEXT column which involved only case changes.
INSERT DELAYED doesn't update timestamps that are given.
YEARWEEK() and options x, X, v and
V to DATE_FORMAT().
MAX(indexed_column) and HEAP tables.
BLOB NULL keys and LIKE "prefix%".
MyISAM and fixed-length rows < 5 bytes.
GROUP BY queries.
ENUM field value
was too big.