I have 2 tabs which were working fine. Suddenly when you select tab1 the content for both tabs 1 and 2 is visible. If you select tab 2 only tab 2 content is visible.
I found this post related to a similar issue: http://forum.webflow.com/t/resolved-tabs-not-working/13232
The solution was to build the tabs again...
as I said this was working fine yesterday and I haven't touched it since