When you type RET in the roster on the contact, chat buffer with this contact is opened. Kill this buffer and type RET in roster again: before first user input roster keybindings are in effect. After user makes any keypress or C-g or even C-h, buffer keybindings become active.
For example, first time you start chat with email@example.com. You press 's' button, self-insert-command is run as expected.
Now you kill buffer and start chat with firstname.lastname@example.org again. You press 's' button, jabber-send-subscription-request is run. If you type C-g followed by 's', self-insert-command is run as expected.
This effect continues untill emacs is restarted.
If you open chat via M-x jabber-chat-with or C-x C-j C-j, it's ok. This bug only encountered when you open chat from roster.
Tested on emacs 24.2.1 with -Q option.