R.T.S.
T/Cook 757 probably pulled back on stand as would be wise to have a butcher's down the exhaust if it failed on engine start. With an internal airflow/bleed offtake problem wouldn't be a surprise if a few flames popped out the back. 737 MEL calls up an inspection of the tailpipe prior to deferrment if memory serves me right. Depends on type but 10 days is quite normal limit for allowing a/c to op without an '$' APU - not v nice this time of year tho....
You ain't seen me right....