Why did my Neocash purchase show up on the wrong account?

The reason why your NeoCash appeared on a different account is that though you had logged out of one of your accounts on http://www.neopet.com, you were still logged into the account on https://secure.nc.neopets.com/get-neocash/buy/amount/.
Note that there are two separate links when purchasing NeoCash.
In order to avoid this issue in the future, please make sure you log off, clear your browser's cache/ cookies and log back in.