Safecracker seems to be executing whether or not a logged_in conditional is true, which affects the action id of other forms. For example, I had something like this:
if logged_in
show safecracker
if:else
show login form
/if
Safecracker was executing even though I wasn’t logged in. So, when I tried to use my login form, the ACT id was wrong, causing the login to fail by redirecting to a blank home page without logging me in. Moving Safecracker to an embed got me around the problem.