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cacophony

A harsh, discordant mixture of sounds; in this context, the disappointment felt can be likened to the jarring realization when expecting something pleasant and finding something unpleasant.

Examples:

The cacophony of emotions rushed through me when I tasted the raisin instead of chocolate chip.
Her laughter turned into a cacophony of groans as she discovered the deception in her cookie.

letdown

A disappointment; something that does not meet one's expectations.

Examples:

That cookie was a real letdown; I had my heart set on chocolate chips.
The anticipated treat turned out to be a letdown when I found raisins instead.

false expectation

An assumption about something that turns out to be incorrect or misleading.

Examples:

Biting into that cookie filled with false expectations left me utterly disappointed.
She faced false expectations when she reached for what she thought was her favorite cookie.

bitter sweet

A mixture of happiness and sadness; in this situation, the enjoyment of the cookie but the disappointment of the flavor.

Examples:

There’s a bittersweet taste in knowing it could have been chocolate chip but was raisin instead.
His bittersweet moment came when he realized the cookie was not what he hoped for.

serendipity gone wrong

A situation where a hopeful or fortunate outcome turns unfavorable.

Examples:

That cookie was serendipity gone wrong; I was joyful until the first bite revealed the truth.
The snack felt like serendipity gone wrong, bringing a raisin instead of sweet chocolate.