Arians pointed out the mistakes to the media on Monday.
"I think there were three really critical errors in the Frisco game -- the interception after we had a really good drive going, the hands to the face when we stopped them and would've still had the lead had they kicked the field goal and obviously that touchdown," Arians explained.
"That touchdown" was the 76-yard pass play to Anquan Boldin that tied the score at seven after three offensive plays by the Niners.
He said Tyrann Mathieu and Antonio Cromartie didn't communicate on a defensive switch -- who would go inside to cover Boldin -- and the rest was history. Absolutely nobody had Boldin covered and he ran into the end zone untouched.
The interception Arians referred to was the one Ryan Lindley threw in the second quarter when the team was already in field goal range.
The offense had driven to the San Francisco 27 on seven plays. It was third down and 13 yards to go and Lindley threw an ill-advised pass that was picked off by the Niners. Arizona didn't get any points out of it and the Niners drove down to kick a field goal. It was a six-point swing and the difference between a 20-17 win and a 20-17 loss.
"(The pick) was a scramble, (Lindley) thinks he can get it to the back for five more yards, shorten that field and field goal when we had the field goal; we could've made that long field goal," Arians explained. "Those are learning experiences."
The other critical play was a hand to the face penalty on Tommy Kelly. It was third and 24 at Arizona's 24-yard line. Colin Kapernick completed a pass for one yard and the Niners would have had to kick a 40-yard field goal. The penalty gave the Niners an automatic first down and, three plays later, Kaepernick finds Bruce Miller in the end zone for the deciding touchdown.
If they make a field goal instead, Arizona still leads 17-16 at the end of the third quarter.
Arians stressed, "If you're going to continue on, you can't make those kinds of mistakes."
With the postseason now here and it is a win or goo home scenario, these are the types of mistakes Arizona can ill afford to make.