gearbox

Good evening all. Just looking for opinions. 1998 4.SE 78,000 miles. Had a new exhaust, cat back fitted today, I know they had trouble getting the old one off because they told me. After leaving the garage tonight, on the journey home, 18 miles the gearbox was making very jumpy changes. No gearbox fault is showing on the dash and it has never showed any sign of this before, is was perfectly alright this morning. So my question is could they have disturbed something or knocked something while replacing the exhaust which could possibly account for this. Many thanks for any opinions.