Ho Ho Ho, spotted a g65816 core bug with F-1 Grand Prix Part 2:

C05940: LDA #$39
C05942: BMI c059a9 ($65)
C05944: ADC $858e65,X
C05948: STX $fea9
C0594B: SBC $950635,X
C0594F: ASL $8a
C05951: CLC
C05952: ADC #$40
C05954: BRK #$aa
00F04C: RTI

[Linked Image from mamedev.emulab.it]

Do you need explainations? ;P