ILLEGAL: q b9-c9 on turn 4:
There was no q on b9. The piece on b9 is a m.
Go back with your browser's BACK button, reload the page, and try again.
For diagnostic purposes, here is the full movelist:
1. P d2-d4 
1... p d8-d6 
2. B c1-f4 
2... p c8-c7 
3. P i2-i3 
3... b c9-f6 
4. B h1-i2 
4... q b9-c9 
5. N g1-f3 
5... n g9-f7 
6. P e2-e4 
6... n f9-g7 
7. M e1-d3 
7... p e8-e6 
8. N f1-e3 
8... q c9-d8 
9. K d1-g1; R i1-f1 // O-O-Og
9... q d8-e8 
10. P c2-c3 
10... m e9-d8 
11. R f1-e1 
11... d9-c9; a9-d9 
12. P b2-b4 
12... p e6-e5 
13. B f4-g3 
13... p e5-d4 
14. P c3-d4 
14... n f7-h6 
15. N e3-g4 
15... n h6-i4 
16. N g4-f6 
16... m d8-f6 
17. B g3-h4 
17... n g7-h5 
18. B h4-f6 // check!
18... n h5-f6 
19. P e4-e5 
19... p d6-e5 
20. R e1-e5 
20... q e8-f7 
21. Q b1-b3 
21... q f7-i7 
22. Q b3-e6 
22... n f6-d7 
23. M d3-g6 
23... resign
In case there is a bug in the code, the settings file can be edited at /play/pbm/play.php?game=Chess8400&settings=Main&submit=Edit