If you schedule a Function that returns a number, the interpreter will schedule the Function to re-run in that many beats.

This will print "5" every five seconds, until you press [Esc] to stop execution.

SystemClock.sched( 5, { 5.postln; } );

To avoid this, you can end your Function with nil, which has been done sometimes through this guide.

SystemClock.sched( 5, { 5.postln; nil; } );

This will print 5 in five seconds, and then stop.