mistake (v.)
early 14c., "to commit an offense;" late 14c., "to misunderstand, misinterpret," from a Scandinavian source such as Old Norse mistaka "take in error, miscarry," from mis- "wrongly" (see mis- (1)) + taka "take" (see take (v.)). Related: Mistook; mistaking.
mistake (n.)
1630s, from mistake (v.).
An error is a wandering from truth, primarily in impression, judgment, or calculation and, by extension of the idea, in conduct; it may be a state. A mistake is a false judgment or choice; it does not, as error sometimes does, imply moral obliquity, the defect being placed wholly in the wisdom of the actor, and in its treatment of this defect the word is altogether gentle. [Century Dictionary, 1897]
Meaning "unintended pregnancy" is from 1957.