While misshifting (shift down instead of shifting up) at 9.000 rpm’s you can crack your valve retainers. It’s almost impossible to see from the top. You have to remove them to check them for cracks.
If they crack and break, you will drop a valve with expensive engine damage.
Ap1 1999-2002 s2000’s has valve retainers who can crack easily. Honda updated them (thicker and stronger) in 2004. We can replace these for you.