Hi
Tested both on PHP 4.x and 5.x
If you chose to use coupong in step 2 or 3 of checkout process efter clicking on Submit button you will be redirect to step 1 of checkout process...
I don't know if this is a bug but I know customers don't like to going trought all step of checkout proces one more time...
Regards
IQAPPS
Confirmed. Totally agree, customer should stay on same page of checkout.