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1.
Partial memory loss, specifically of an isolated event.

Very conveniently (and in some cases thankfully) tends to happen to people after a night of drunken revelry.

Also a very common plot element of bad romance novels and bodice rippers.

Which is not to say that it's not used in books and movies with an actually good plot. "Eternal Sunshine of the Spotless Mind" comes to mind.
"The way those deaths occurred, any reaction is possible including lacunar amnesia."
—And The Children Shall Lead
by edenz January 29, 2011