In Matthew 13, the SAB asks the question, "Did Jesus have secret teachings?" I think this is a question that ends up being simple and complicated at the same time, depending on how you look at it. On the simple side, I think that what Jesus is saying in John 18:20 is that he doesn't have a hidden agenda, not that he'd never said anything less than 100% public, which is clearly not true. On the complicated side, I've been fascinated for years with the question of what the purpose of Jesus' parables were, because it's far from completely clear. While I think most people think of them as little illustrations to help people understand spiritual truths, Jesus says outright to his disciples here and in similar passages in Mark 4 and Luke 8 that he's using parables to hide truth. And really, some of Jesus' parables are pretty twisted, and it's far from clear what the exact point is that he's trying to make. We'll get to at least one weird one in this chapter.
(The issue of whether there has ever been a righteous person was one that I addressed here, saying that there are different kinds of righteousness.)
In verses 31-32, Jesus tells the parable of the mustard seed, which is definitely one of his odd ones. Now while there are smaller seeds than the mustard seed (as the SAB points out) and so this might be considered a scientific error, there's something odd to consider here. While the mustard seed may have been the smallest one known to or used by first-century Israelites, as the SAB also points out, the mustard seed doesn't grow into a tree, and this would have been fairly common knowledge to Jesus' audience. So I don't think Jesus is trying to make a scientifically accurate statement here, but rather is trying to paint a picture of a bizarre miraculous occurrence. What this means, I can't say for sure, but I think most people have suggested that it's an illustration of how the teachings of a single man managed to grow into to world's largest religion.
As the SAB says, verse 35 is a misquote of Psalm 78, but I think it's just 78:2, not verse 3 as well. While it's a misquote, it's fairly close. The psalm says "dark sayings", but it's a translation of a word more often translated "riddles" meaning secrets or mysteries. The psalm doesn't say anything about "the foundation of the world" but does say "of old" using a Hebrew word meaning "of ancient time". (The Greek word translated "foundation" almost never appears in the N.T. without "of the world", for whatever that's worth.)
The parable in verses 47-50 provokes a couple questions from the SAB. Is anyone good? I'm going to go with "Yes" on this one, although it should be noted that "good" is a comparative term. The Isaiah verse really is about righteousness, an issue with a response I linked to above, while the Mark verse is an oddity that has something to do with (if I recall correctly) a belief that the Pharisees had about God. Has there ever been a just person? I think again, the answer is yes. I'm not sure what the deal is with the Ecclesiastes verse in this case. First of all, it seems to be describing not someone who is "just" but someone who is "righteous", which suggests a questionable translation (although that may open up a bigger can of worms). Secondly, I think that the book of Ecclesiastes is prone to hyperbole more than most books, and Solomon may be implying that, just or righteous, the sort of person he's describing is incredibly rare. In case it's not clear, I think the distinction between "righteous" and "just" is that the latter would describe someone who does the right thing while the latter describes someone who believes in doing the right thing. Such people are, in my opinion, not so rare as Solomon suggests.
Lastly, the SAB notes the absurdity of Jesus being largely rejected in his hometown. I don't think this is necessarily so absurd, as it may be true that someone who knew a famous person as a child might have a hard time fathoming their rise to fame. Jesus may have had a largely unremarkable childhood, seeing as virtually nothing of it is recorded in the Gospels. And so long as we're here, I would say regarding Mary being a lifelong virgin that while I don't believe it personally, I don't think it's an unreasonable stretch that these "brethren" may be half-brothers or cousins, and thus the Catholic stance on Mary's virginity may hold.