Wikipedia says that "in RNA, thymine is replaced with uracil in most cases." I'm curious what are the cases when this does not happen? Does this ever occur in normal functioning, or is it an error? Do T and U ever appear in the same strands of RNA together, or does a single strand just use T or just use U?
tRNA molecules contain a T in the T arm. I believe that this results from post-transcriptional modification of a U by uridine methylase.