Yes, this is a strange behaviour we can found in fact when the view that holds the UIActionSheet doesn’t goes to its end.
In fact, it is visible especially on a child-view of a tabbarController.

If you call an UIActionSheet like this :

UIActionSheet *anActionSheet = 
[[UIActionSheet alloc] initWithTitle:@"Attention" delegate:self cancelButtonTitle:@"DO IT" destructiveButtonTitle:@"DON’T" otherButtonTitles:nil];
anActionSheet.actionSheetStyle = UIActionSheetStyleBlackTranslucent;
[anActionSheet showInView:self.view];
[anActionSheet release];

You’ll see that the "DON’T" button is only clickable on its first 5 or 6 top pixels…

The good correction has to be made to the "showInView" part, replace it with :

[anActionSheet showInView:self.parentViewController.tabBarController.view];