Overstimulated Ocean

Normally, I'm pretty savvy about handling cats, and usually avoid getting bit. But not always! Ocean is a shelter cat that recently nipped me without any obvious warning. Fortunately I was recording my interaction with her, so I reviewed the video afterwards. I saw subtle potential warning signs.
[SPOILER ALERT: Watch the video before reading any further if you want to try detecting warning signs on your own. The video will be shown twice: first unedited, then with comments about the possible warning signs I see.]
Ocean shook her head at one point, possibly because the petting was beginning to irritate her and she was trying to "shake me off." She also reached out a paw towards my hand, again possibly trying to get me to stop. However, I have seen both of these actions in cats that haven't bitten me, which leaves me unsure if they were warning signs. (My husband thinks another warning sign is that Ocean's purring changes right before she bites.)
As a cat behavior consultant, I know that cats always have reasons for their actions. What was the reason Ocean bit me? Well, I had given her catnip earlier on my rounds, so perhaps she was feeling a bit wild from its effects. Otherwise, I can only guess. Being cooped up in a shelter creates pent-up energy in a cat, as does being petted, and so the two could have wound her up too much. Another reason is that if their warning signs get ignored, cats can learn that biting is the only way their needs get met. Ocean got adopted before I could figure out her trigger, so I'm left with only guesses.
What warning signs do you see? Please share your insights!