Pre-recorded and live (or close to live) programmes may contain subtitles. Generally subtitles are transmitted problem free but occasionally a viewer may notice that subtitles for a particular programme are out of sync with the picture.
The majority of programmes have subtitles created in advance of when a programme broadcasted. To create subtitles, a sub-titling technician will use the audio and picture to correct match them in sync. If the subtitles are out of sync when the programme is broadcasted then either the technician has misjudged the timing for the script and visuals or the script has been changed/amended prior to or after a programme has been subtitled. Sometimes the scripts start out correctly and in sync with a programme only later to have wording updated in the middle of the script etc.
For Programmes that are live (e.g. News) or recorded close to live (e.g. Mock The Week), subtitles are generated by a subtitle technician live, as the programme is being transmitted. With live subtitles there is often a delay and this will be noticed either watching live or via the catch-up service.