Crawl issues I am sure but also user experience issues. Google is sensitive to sending visitors to sites where metrics indicate users do not, like bounce rates etc. I don't use twt but if it is the case you have the be logged in to see anything now, a non-logged in user will click a link from Google hit a login page, and use the back button. I would assume Google will see that as a bad search result and use it less.
Crawl issues I am sure but also user experience issues. Google is sensitive to sending visitors to sites where metrics indicate users do not, like bounce rates etc. I don't use twt but if it is the case you have the be logged in to see anything now, a non-logged in user will click a link from Google hit a login page, and use the back button. I would assume Google will see that as a bad search result and use it less.