On the day Manchester City sealed the title, its former goalkeeping prospect had a moment to forget as his poor touch granted midfielder Pritchard the simplest of finishes 10 minutes after half-time.
The goal cancelled out Nathan Redmond's first-half opener and secured Jan Siewert just his fifth point in charge of the Terriers, who concluded their second Premier League campaign at the foot of the table.
Southampton's inability to make its superiority count will only strengthen Ralph Hasenhuttl's resolve to reinforce a side that ended the season on a five-match winless streak.
Siewert dropped outgoing goalkeeper Jonas Lossl in favour of Joel Coleman and the 23-year-old's first real act was to gift away possession in a dangerous area.
Shane Long failed to take advantage, shooting over the crossbar on the turn, but Redmond offered no such mercy in the 41st minute.
The winger cut across the top of the box and curled a brilliant finish beyond Coleman's despairing dive.
James Ward-Prowse tried to mimic his team-mate moments after the interval, but Huddersfield's deputy stopper did well to tip a fine attempt onto the bar.
Southampton's course for victory seemed set until Pritchard pinched the ball off a dozing Gunn and restored parity in the 55th minute.
The mistake took the air out of the home crowd and, though they continued to do the bulk of the pressing, a second failed to transpire for the host.