A Bug

(entomology) An insect of the order Hemiptera (the “true bugs”).
Any of various species of marine or freshwater crustaceans; e.g. a Morton Bay bug, mudbug.
(colloquial) Any insect, arachnid, or other terrestrial arthropod that is a pest.
(colloquial, US) Any insect, arachnid, myriapod or entognath.
(chiefly computing and engineering jargon) A problem that needs fixing. Synonyms: defect, glitch
A contagious illness; a bacterium or virus causing it
(informal) An enthusiasm for something; an obsession
(informal) A keen enthusiast or hobbyist.
A concealed electronic eavesdropping or intercept device
A small and usually invisible file (traditionally a single-pixel image) on a World Wide Web page, primarily used to track users.
(broadcasting) A small, usually transparent or translucent image placed in a corner of a television program to indicate what network or cable channel is televising it
(aviation) A manually positioned marker in flight instruments.
A semi-automated telegraph key.
(obsolete) Hobgoblin, scarecrow; anything that terrifies. [late 14th c.–early 17th. c] Synonyms: bog, bogey, bogle, boggle, boggard, bugbear
(chiefly LGBT, "the bug") HIV.
(poker) A limited form of wild card in some variants of poker.
(paleontology, slang) A trilobite.
(petroleum industry, slang, dated) Synonym of oil bug
(slang, horse-racing) A young apprentice jockey.

Adjectives often applied to “bug”: major, minor, serious, critical, nasty, annoying, important, strange, stupid, flying, silly.