What mistake am I making when deleting page types/classes? If I create ExamplePage.php, try it out in the CMS and then remove it, db/build?flush=1 gives me:

FATAL ERROR: DATABASE ERROR: Couldn't run query: ALTER TABLE 'SiteTree' [or 'SiteTree_Live'] CHANGE 'ClassName' 'ClassName' enum('SiteTree','Page','BlogEntry',[etc. ExamplePage is not included]) character set utf8 collate utf8_general_ci default 'SiteTree' | Data truncated for column 'ClassName' at row 4

Problems with settings, e.g. in MySQL? Have I missed something about class deletion? The problem doesn't occur every time I delete classes, but most of them.