When making tea/instant coffee/hot chocolate etc. on Shabbat, it is supposed to be done in three vessels, so that the boiling water cools off and doesn't cook the powder. The urn is the first, then there's an intermediate cup for pouring, and then it gets poured into your cup (teabag goes in after). (See Shmirat Shabbat C'hilchatah 1:57)
If someone forgot to do the cli shlishi, instead adding the tea to the second cup after the water was put in it, can the tea or whatever still be drunk?