No experience gain
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Certain monsters in Crawl are flagged as having no experience gain For the most part, these are simply monsters that are completely inert, such as plants and toadstools, but some monsters that only appear as part of special summoning spells, such as snaplasher vines, are also included in the category. As the name suggests, killing said monsters never results in any experience gain (and by extension, any skill gain).
For a complete list of such monsters, see