At the Stadio di Monigo, Priestland kicked ten points but was outdone by Treviso's Alberto di Bernardo - the Italian showed nerves of steel to kick a last-minute 50-metre penalty to make the score 22-20 and record Treviso's second win of the season.
Priestland scored half of his team's points and looked to have buried Treviso just after half-time when he slotted a penalty to put his team 20-5 up.
Winger Andy Fenby had also ran in two tries in a first-half dominated by the Welsh club, but following the break Di Bernardo rounded off a superb fight-back with his last-gasp penalty.
Likely to be challenging Priestland for the Wales shirt will be Dan Biggar, and the 22-year-old certainly impressed with 15 points for the Ospreys in their 30-15 win over Munster.
The reigning champions have had a shaky start to their title defence but were superb in their well-deserved win.
Most of Munster's response came from the boot of Ireland's Ronan O'Gara who showed he can still kick with the best of them as he claimed 12 points, but it was not enough and the Welsh team ran out comfortable winners to record only their second win of the season.
Elsewhere in Friday night's games Wales received a blow to their RBS 6 Nations ambitions with influential flanker Dan Lydiate breaking an ankle in Newport Gwent Dragons' 32-12 win over Edinburgh.
Reports suggest Lydiate may yet be fit in time for the later games in the Six Nations campaign, but it is a big blow to the defending champions.
Meanwhile Ireland winger Tommy Bowe marked his return to Ulster action with two tries as they beat Cardiff Blues 48-19 to go top of the RaboDirect PRO12 table.