You may be cursed, or rewarded, by deity-level beings.
Disjunction comes from D&D's D20 SRD, which in turn is from the D&D Mordenkainen's Disjunction. It has had the "attention of a powerful being" clause since its appearance in AD&D 1st edition's Unearthed Arcana. The following examples are taken from D&D products, where the spell originates.
Cursed, by a deity
The D&D adventure module Axe of the Dwarvish Lords, p.9, describes the effect of this spell on the Axe of the Dwarvish Lords:
In addition, Moradin is 95% likely to visit a curse upon the spellcaster. The curse can have any one of the following effects (DM's choice):
The suggested curses are a disfigurement which weakens the character and makes them hated by all dwarves, an earthquake which surrounds the character whenever they cast a spell (save negates), or an ally's weapon breaks whenever they cast a spell in combat. The only way to remove the curse is to receive an atonement from a priest of Moradin, or to do a great service for a deity opposed to Moradin.
Note that even so, the axe only vanishes, but is not destroyed. Page 183 of Axe of the Dwarvish Lords asserts that Disjunction can destroy four minor artifacts called the Great Tools, which share the same creator as the Axe. No divine backlash is specified for these items.
As per the rules in Deities and Demigods, many deities become automatically aware of certain things within their domain, such as artifacts related to them. Destroying an item sacred to Moradin, for example, would very specifically draw Moradin's notice, as may the destruction of artifacts significantly affecting the dwarves. Deities are therefore a prime candidate for noticing Disjunction use.
Unspecified harm, by a deity
Dungeon magazine #124 adventure Chambers of Antiquities, p.87, includes a sarcophagus containing a mummy cleric of Wee Jas. A minor artifact, it can be rendered nonmagical with Disjunction, but doing so would be:
an act that incurs the wrath of Wee Jas.
The exact penalty is unspecified.
Rewarded, by non-specific "powers of good"
Book of Exalted Deeds, p.120, suggests that:
The powers of good smile upon those who rid the world of great evil by destroying a major artifact. A party of good characters who destroys an evil artifact can request a miracle at no cost.
It also suggests that destroying an evil artifact while standing within a consecrate or sacred place can help protect against the backlash, although it does not specifically protect against the 95% chance of being noticed by a powerful being.
Unspecified danger, by a powerful titan
Dungeon magazine #134 adventure Into the Wormcrawl Fissure, p.62, describes a lillend who disjuncted a Staff of the Magi ownwed by the titan sorceress Kelastis, and was thereafter forced to flee to the Material Plane to avoid that being's wrath. This is more literal and direct than usual, but it does describe a situation where using the spell invokes a powerful being's attention, because you literally broke a thing they own and now they're angry.
Summoning a demon, by Demogorgon
Dungeon #147's Into the Maw, p.81, describes the affect of using disjunction (or any other means) to destroy an altar to Demogorgon, a demon prince:
Destroying the altar removes the forbiddance and sympathy fields, but also immediately alerts Demogorgon, who immediately sends a molydeus demon named Zarvab to investigate. The demon is enraged to discover the condition his master's prison has fallen into, and once he deals with anyone who remains in this area, begins a brutal crusade to cleanse the site of every living thing.
Overall, there seems to be a pattern here. Destroying an artifact commonly angers a deity, demon prince, or similarly powerful being who created, owned, or had a vested interest in that item. However, you may also be rewarded by an equally powerful being who hated the item and wanted it destroyed.