[ShadowHeart]

...

He was an experiment, after the Nibelheim incident.

After Cloud threw Sephiroth into the Lifestream, Hojo arrives to the burnt out village and collects all survivors for experiments. Cloud and Zack are being kept in the basement of the Shinra Mansion and experimented on, they are injected with Jenova cells and so on. Zack is a strong person so he doesn't respond to the treatment at all, while Cloud flips out completely. Eventually they manage to escape but Shinra soldiers catch up just when they reach Midgar and they shoot and kill Zack, but they leave Cloud be as they think he's harmless due to the Jenova treatment (if you go to the Shinra Mansion basement you can watch a sequence about this and you'll understand why they thought Cloud was harmless). Anyway, Cloud assumes Zack's memories and story as his own - as in really believing that's how it was - and "forgets" what really happened until Tifa helps him remember on the second disc in the Lifestream.

So as I said, yes, he was an experiment, he was injected with Jenova cells, but that was after the Nibelheim incident. Cloud throwing Sephiroth into the Lifestream was completely his own doing, he wasn't "enhanced" yet.