A recently published work by Peter Husen and Ilia Solov'yov of the QuantBio-goup reveals, that oxygen molecules can bind at the Qo-site of the bc1 complex. This is likely leading to the formation of superoxide, which is toxic for the cell and may even be an important factor in ageing.
You can read more about the paper on the SDU website, where it was recently featured.
The paper, Spontaneous Binding of Molecular Oxygen at the Qo-Site of the bc1 Complex Could Stimulate Superoxide Formation, can be found here.