His team trailing by nine points, he bangs over the penalty that brings them within range; hooray, he's a hero. Then he misses the reasonably simple conversion of Henry Pyrgos' try: boo, a loser. Then a second penalty chance arrives, up he steps and over goes the ball: hooray again, confirmed as the hero of the hour.
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It is a sequence that uncannily mirrors his whole season. On a high as he secured the starting spot at fly-half in the Six Nations; on a downer as he finished the Glasgow season so far out of the picture that he could not even make the match-day 23 for the RaboDirect Pro12 semi-finals or final; now back in the international fold and back winning games for Scotland. Up, down, up again, but on a much longer timescale.
He admits it was a bit of a howler to have missed the conversion, especially since he had converted Stuart Hogg's try in the opening minutes from a much more difficult position. One mildly mitigating factor: the uncertainty.
There was no scoreboard at the ground - the only visible record was the scoreline in the corner of the television screens a good 100 feet above the pitch, which was far to small to read without binoculars. So when Weir stepped up to take the kick having lost track of the scoreline, he was distracted by having to ask what it would mean.
"I was asking the guys upstairs what the score was, and they were saying if I kicked it then we would be a point up. I didn't know the pressure was on, it was just a bad kick from me," he said. "I knew exactly what I had done wrong and as a result I hung it out by the right post.
"For the forwards to draw that penalty from the drive was great and I knew exactly what I had to do to get the kick over. It was always going true so I had a wee turn of the hips and turned back giving it the wee cheer; it was a good feeling."
It really was one of those games where a reasonably solid final 10 minutes hid a lot of Scots' blushes. They started well, finished well, but in between was another blunder-fest with nothing much going right as missed tackles, giving away penalties and getting a doing in the breakdown, put them on the verge of defeat.
Still, they found a way out of it and Weir says that is what matters. "Vern [Cotter, the head coach] said to have won these games [against Argentina and Canada] by two and one points is going to lift our confidence. Though that first half was a long way away from perfect - the Argentine breakdown skills were terrific - we did not help ourselves with unforced errors, a few kicks out on the full and all that. Hats off to them in the first half, we could not really get into the game.
"We have only had a short time with Vern, the guys coming in have not had a camp with him to understand what he wants. There were a lot of questions this week: What do you want to do here? What do you do in this scenario? So I am delighted we came away with a win. It stands us in good stead to go to South Africa and hopefully do a job on them.
"We know they are one of the most physical teams in world rugby with the size of their pack, the power of their backs, they are really good. We will have our work cut out."
To add to the complications, Scotland have now lost Kieran Low and Blair Cowan, the London Irish back rows banned by the English Premiership from taking part in this week's game, and have had to dig deep to find replacements. Tyrone Holmes has at least played reasonably regularly for Glasgow Warriors, though not in any of the big games, but Adam Ashe has been injured for a large part of the season. He should be match fit having been playing for Lincoln University in Christchurch, New Zealand, after winning the John McPhail scholarship to train there, and is a specialist No 8.
On the up side, they have also been able to draft in some experience with Euan Murray an undoubted boost. After settling his future by signing for Glasgow last week, he is now free to travel.