I read that USSR was boycotting UN due to Communist China not being recognized and they were blindsided by the outbreak of the Korean War. Kim Il-sung asked Stalin's permission to launch offensive and was most likely sharing their plan with USSR. How could USSR be unaware that a UNSC vote was being taken on UN's intervention in Korea? When the vote was cast without them, did they protest? Did USSR intentionally let UN and thus US get involved in Korean war?
The Soviets assumed that their boycott would be taken as a veto. When it wasn't, they came rushing back and tried to claim their non-participation made the relevant UN resolutions and thus the UN military presence in Korea illegal. Nobody bought it, though.