I have not tested this extensively to see every single possibility, but Y'shtola seems to have stolen Urianger's line in the Tower of Babil just before the first boss.
Specifically, she says, "He doth not appear to have been tempered." Given that Y'shtola's speech patterns don't include the word 'doth' but Urianger's do, I can only assume she was temporarily possessed by him in the various instances I've seen her say this.
Steps to recreate:
1) Have Y'shtola or her avatar in your trust party, without Urianger. (If she says it when Urianger is there, I wouldn't know as I have not run it multiple times with them yet.)
2) Approach the first boss.
3) Y'shtola, overcome by the spirit of Shakespeare, utters the line in question.
4) The other trust members act like nothing is wrong.
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