Happened to me once , at BGI I was booked to GEO , arrived at the gate late unaware they were loading two flights ( it was LIAT !) I followed the man in front , sat down on my allotted seat , if there were announcements I did nt hear them , but once air born I noticed we were heading in the wrong direction I enquired with the passenger next to me who told me we were going to SVD; he called the stewardess and a discussion ensued between her and a number of the passengers as what to do, - the solution it seemed was to continue on the flight to the end destination ,POS where someone knew that BWIA had connecting to GEO : this was all organised from cockpit and somewhat red faced I arrived some two hours late. - This was some 20 years ago and it was the Caribbean