Can't work out if I'm being dumb here, but I'll take a chance on it.

a fairly normal serial int routine ...

if (TI){
SBUF = *NextTxChar;
}

if (RI){
LastRxChar = SBUF;
}

Looks OK? But if TI and RI should happen to happen on the same m/c cycle (not likely, but possible), the tx part will trash the incoming char.

So the order should be reversed. Am I right?

(Sometimes engineering feels like a course in advanced pessimism.)

Thanks

Daniel