。
She accidentally put salt in the coffee.
Literal
She [topic-は] mistakenly coffee [in-に] salt [object-を] put.
間違って (the te-form of 間違う 'be mistaken') functions adverbially as 'mistakenly / by accident' — Japanese uses te-forms productively as adverbs of manner, especially with verbs of cognition and error. The sentence plays on the universal kitchen joke that salt and sugar look identical: small white granules in matching containers, almost impossible to tell apart by sight. 塩 ('salt') and 砂糖 ('sugar') even pair as a fixed contrast in Japanese (塩と砂糖), the way English pairs 'salt and pepper.' に here marks the destination of the action, the container she put the salt into.