Gremlin

Noun

 * 1) A mythical creature reputed to be mechanically inclined and mischievously inclined to damage or dismantle machinery.
 * 2) By extension, any mysterious, unknown source of trouble or mischief.
 * We rechecked everything, and we suspect gremlins in the database.

Thesaurus
Ariel, Befind, Corrigan, Dingbelle, Fifinella, Finnbeara, Hob, Hobgoblin, Mab, Oberon, Puck, Titania, bad fairy, bad peri, banshee, brownie, cluricaune, deviling, devilkin, diablotin, dwarf, elf, erlking, fairy, fairy queen, fay, gnome, goblin, hob, imp, kobold, leprechaun, little devil, ouphe, peri, pixie, poltergeist, pooka, puca, puck, pwca, sprite, sylph, sylphid, tokoloshe, young devil

Etymology
Possibly from Old English grëmian ("to vex", "to anger"), related to German grämen and Swedish gräma ("to grieve").