Lately I was looking into the Google Mail Messenger being able to sign you in to AIM (and thus ICQ) accounts.
The strange thing is that it said my credentials to be wrong.
As you probably know, ICQ can only use the first 8 characters of the password. For some reason I had stored a 9 character password, which gets trimmed to 8 characters by every popular software (including the original clients).
However, not so for Google Mail which tries to sign you in with all you typed and thus fails. So, if you want to sign into ICQ using Google Mail and it complains, remember to only type the first 8 characters of your password.