I once set up my Rio 300 tank without a lid, as the support in the tank had broke so I had to remove the lid to repair it. In there at the time were 3 oscars and 4 large silver dollars. Next to the tank was my budgie parakeet cage. While doing some maintaince in the cage the budgie escaped. It flew into the wall and landed in the tank!
I could hear the budgie screeching so I had to quickly remove it, but the oscars were bigger than the bird so I thought for the worst. I got it out and it had lost most of its tail feathers, but survived. So now I never leave the lid off.