Gotta +1 on this.
Really really annoying.  Happens every time I want to login and make a quick post on a thread I'm following.  The worst bit is I then lose whatever I've just typed out, the message input doesn't get restored on a back button press, which I've noticed some forums to manage.   I do understand this is to protect the security of the forum from DOS etc, but this must be solvable surely.  Maybe the database needs two write flags a last_login_in flag and a last_post_flag in order to differentiate the two completely different actions.