Hi Mike this small test in BCD mode causes a python exception:

.assemble 0
$0000  f8        SED
$0001  18        CLC
$0002  a9 ff     LDA #$ff
$0004  69 2f     ADC #$2f

Error: <type 'exceptions.IndexError'>, list index out of range:
 file: [snip]py65/utils/conversions.py line: 21

I wouldn't be surprised if the ADC and SBC code to handle decimal mode need some attention anyhow, so this issue can perhaps act as a marker for that.

Cheers
Ed