Unstable means that Prettier produces a different result on second run. First run:

Prettier 1.14.2
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--parser babylon
--no-semi

Input:

x
;/* comment */ (error: any).response = response

Output:

x
/* comment */ ;(error: any).response = response

Second Output:

x
/* comment */
;(error: any).response = response

Expected behavior:
Comment should stay on the same line, or it should have been shifted in the first run. Slightly related issue in #1510.