When you click on the menu bar icon you expect it to act in exactly the same way as the dock icon does.

It mostly does, but if you hide the Tweetie window by clicking the red close button, Tweetie retains focus. This means that if you suddenly want to show it again you click the menu bar icon and it returns focus to the next uppermost app. You have to click it a second time to restore Tweetie.

Also, when restoring Tweetie from closed, or hidden (pressing Cmd-H) with the menu bar icon it sometimes doesn't give focus to the Tweetie window so you have to click on it to use the keyboard to move up/down.