I installed Recent Custom Posts (2.1) so that I could have a widget area of Exhibitions in the Gallery theme.
I placed the widget in Footer Widget Three and configured it to show custom post type Exhibitions.
After reviewing the page (looked OK) I went back to the Widget admin screen and saw that the Footer Widget Three area had an error:
Fatal error: Call to undefined function preg_filter() in /[my site}/wp-content/plugins/recent-custom-post-type/recent-custom-post-type.php on line 191
I tried uninstalling and reinstalling the plugin - same deal.
I also noticed that while the plugin is activated the widget admin screen doesn't show anything past the Third footer widget area (see attached screen shot) and the expand/contract arrows don't work.
With RCP deactivated there isn't anything in Footer Widget Three and everything works as it should.
Gallery Theme 1.5.0
Membership Premium 3.4.2
WPMU Dev Dashboard 3.2.3
I tried deactivating all non-WPMU plugins but it still happens.