It's that the blade was poisoned in the first place. Like for what? Were they expecting to run into Jamie and Bronn? They'd heard that they were travelling to Dorne, but had no reason to think they'd be in the Water Gardens at the time. If not, were they trying to poison the personal guards of Prince Doran that were on site and the might run into while trying to kill/kidnap/whatever the fuck to Mycella at the time? Was the posion for Mycella?

It's another poorly thought out story beat in a really poor storyline. But at least we got a great pair of tits.