Now seeing it in 7 also.

Just had it happen without hibernating. It's hard to catch what it's doing but in this case for a tab I closed in the browser I think Sidewise closed the wrong tab in its list and left the one it should have closed orphaned.

I've been getting the exact same thing for weeks now too, but only seems to happen with my Win 10 machine. It may be occurring after I hibernate and return. It's like Sidewise gets out of sync with recent and new tabs. If you close a recent tab it doesn't close in Sidewise and vice versa. You create a new tab and it doesn't appear in Sidewise.

Thank you for looking into this. I guess people who had Chrome update automatically didn't have this problem. I've liked Sidewise since I started using it, but being without it for even a short time has made me realize just how much I depend on it. So my thanks also for creating such a useful extension.

Free, although donations are accepted. I have donated to this app's development.

Would've been nice if the developer had made some attempt to help, but I found the solution. Had to update my browsers (Chrome derivatives) to latest version. I never had to do this before, neither is it mentioned anywhere that I have seen that this might be necessary after an update. Hope this helps others who have the same problem.