Apparently back in the day they were seriously considering ClF5 as the next generation oxidizer for rocket propellants. Implying that the rocket exhaust would to a large extent consist of HF and HCl. Yikes!
Equally funny is where someone thought that it would be a wonderful idea to burn mercaptans in a rocket, which wasn't quite as toxic, but caused the test site to become a no-go zone just from the unbearable stench.
As described in Clark's book, ClF3 was discovered first (before WWII actually). But sometime later someone managed to synthesize ClF5 as well, which had slightly better performance as a rocket fuel oxidizer. Although it retained all the other fun properties of ClF3.
Equally funny is where someone thought that it would be a wonderful idea to burn mercaptans in a rocket, which wasn't quite as toxic, but caused the test site to become a no-go zone just from the unbearable stench.