Nothing to see here.
Told to wait on the runway. Tedious holding the brakes on with feet for more than 20 secs or so, so park brake goes on, nothing wrong with that.
Take-off clearance given; 99.9% of the time, we release the park brake and apply thrust etc.
Once in a blue moon, we do it in the wrong order and forget the park brake....oh bugger !; park brake off, "that's better" and away we go.
A bit like in the SIM when you are setting off for the umpteenth take-off but have forgotten to input a flex value on this occasion. Engines spool up but no flex available.....huh?, oh yes, bugger! select TOGA and away we go.