According to complaints across the Web, the co-op feature of Borderlands is currently broken on the PS3 console.
Symptoms of the bug include freezes when opening the public game list and a general lag-filled experience.
Gearbox Software Engineer Scott Velasquez has posted the following in the Gearbox Community thread on this issue, to quote:
Update on the North American PS3 Friends Interface issue:
We've identified the problem and are working on a fix. Believe it or not, the friends feature was heavily tested by Gearbox and 2K. However, this particular issue we found could have never manifested itself in our or 2K's test environments.
We'll find out very soon if the issue exists for the PS3 International versions of the Borderlands. If it does, we'll hopefully have a fix implemented before it affects those customers.
We are doing everything we can to get this problem addressed as quickly as possible. Thanks to everybody who provided us with tons of information.
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