What undesirable flavor does diacetyl impart to beer?

Diacetyl is a compound produced during fermentation, often a byproduct of yeast metabolism, and it is indeed associated with a distinctive buttery flavor in beer. This flavor can be described as reminiscent of butterscotch or buttered popcorn, which can be quite undesirable in most beer styles. While certain styles may tolerate or even showcase some butter-like flavor, such as in some British ales, high levels of diacetyl are generally not accepted in most other styles where a clean flavor profile is preferred.

The other flavor options mentioned do not accurately describe diacetyl's characteristics. Sourness is typically associated with lactic acid and other bacteria, bitterness comes from hops and certain malts, and fruity flavors arise from esters generated during fermentation. None of these relate to the buttery flavor profile that diacetyl contributes.