I accidentally typed my root account password on my google account login, I know google keeps their website quite safe but this question came in mind: Is there a risk if you accidentally type in another password in another websites login page?
I would guess the risk in this particular situation (Google services) is quite minimal, but you're always taking a chance when you enter the password of another account into a third party service. This is because it is unknown as to what they do with that data. They could do nothing, or they could data mine, sell, attempt to log in to other known accounts, and/or otherwise run analysis on passwords entered there.
There are a few things to consider, for example if the username of the two accounts in question are not the same it adds a layer of obfuscation to the mix. What username would they use to log in to your account on another service with out more information about you and your online presence? How would they find the right service to use the correct combination of username and password?
When to be worried? I would worry if I were using the same username on multiple online services where I didn't particularly trust their origins (think sites like hackforums.net). If you're doing something like this it could be easy for a black hat owner of the service to do some recon on your username by way of Google "hacking" or namechk.com and trying your passwords on your accounts.
Yes it could be a risk as you've effectively given your password to a third party. If you don't trust them you should change the password on the site where the password was from. Even if you have a different username, it might be possible to determine which account is yours on another site by other means.