Microsoft will be performing scheduled maintenance that will affect Xbox 360 titles and services tomorrow, June 20th.
The maintenance will run from 4 a.m. ET until 6 a.m. ET and will also impact Xbox 360 backward compatible titles on Xbox One.
On 20 June, maintenance on Xbox 360 titles & services may interrupt gameplay including back compat. More info: https://t.co/XnTd43iz4b
— Larry Hryb @ E3 ⌨️ (@majornelson) June 19, 2017
Microsoft notes that Xbox One owners who are playing an Xbox 360 title during the maintenance may be disconnected from Xbox Live one or more times. However, Xbox One games and services will be unaffected by the maintenance.
During E3 last week, Microsoft revealed that it is extending its backward compatibility program to include original Xbox games, starting with Crimson Skies and Fusion Frenzy. The company did note that the original Xbox side of backward compatibility will be no where near as big as the Xbox 360 side.
Source: Major Nelson