Google+ went live some time ago, and pretty much ever since, some webmasters have occasionally encountered a red button with a exclamation mark when submitting content.
The Google plus button only turns red when a error occurs, according to the official red-button help page. But sometimes it doesn't appear like any error occurred, which makes webmasters speculate that Google also shows the famous red button when it decides that you have submitted to many pages from the same domain.
Why is the Googles red button not pressing?
Reports so far suggest to try and submit a page from another domain, and then try and submit whatever you originally intended. If nothing else, you can even +1 the red button help page, which may just alert Google that this occurs pretty often at the moment.
Of cause they likely won't change the behavior if its intended to reduce spam submissions.
Google not alone
When Stumbleupon thinks that someone is submitting to many pages from the same domain, it simply shows a blank page, and wouldn't even show an error message. Webmasters have also been guessing about the caurse in this case, and pretty much everyone agrees that its from submitting to many links from the same domain.
Other social sites are more user-friendly, taking care to inform users about the cause of common errors. The more general error messages are only displayed when something really unexpected goes wrong.
Twitter used to not show errors when you hit the daily following limit, but is not showing a general error telling users that they have hit a follow limit.