I am the author of several plugins, which can be used as widgets. All of them work fine in 2.6, but in 2.7, when I add a widget to the sidebar, then click "save", it disappears. But here is the interesting part. If I then select "show unused widgets", then the widget is listed there, and when I add it and save, it works just fine. I spent quite a few hours looking at the text widget to figure out what I needed to change, but I never figured it out.
The plugins I am referring to are:
They are all fairly similar. Each one has a collapsXWidget.php file, which handles all the widget stuff. In my debugging I did determine that it seems to be those functions which are causing the problem, but I couldn't nail it down. I am using the wp_register_sidebar_widget instead of the register_sidebar_widget, because the wp_ version has some extra parameters which I want to use. But, as they note in the documentation, the wp_ functions are more likely to change between versions.
Any help would be greatly appreciated.