Even powerful poisons have a shelf-life. One of the young men who failed to kill Archduke Franz Ferdinand back in 1914, drank a bottle of poison that was professionally compounded. He then tossed himself into the river. But the poison had expired. Rather than killing him, it simply made him violently ill. He survived, was fished out of the river by the Authorities, and arrested. One of his co-conspiratores was successful later in the day.
Ah that brings back memories of high school history class...
If I remember right, the guy you're thinking of was Princip(?), can't quite remember the first name; Gavrillo I think. Anyway, he was in fact the successful assassin, though somewhat by stroke of fortune. He was a member of the "Black Hand" society who were out to assassinate the archduke, but Princip lost his nerve. However, the archduke's driver happened to pull the car up directly in front of the reluctant hitman, who proceeded to pop him with his pistol.
He then fled to a bridge over a river, and bit down on his glass cyanide capsule, then hurled himself into the river. He was unlucky on two counts: (1) the "river" was more like a sewage drain trench from the city, and not very deep, and (2) the incorrect mix of cyanide made him violently ill rather than killing him. So, he found himself up to his waist in sh.... er,
sewage, and puking his guts out, as my history teacher re-enacted with sound effects like BBLLLLEEEEEUUUURRRRGHHHH!!!!!!
Which is when the authorities caught up with him. I bet there were no eager volunteers to fish the guy out. A rather fitting situation for the guy who kicked off WWI, which led directly into WWII.
My memory may be a little off because it's been about a decade since I was in that history class