Mustard was a weed in the fields of the Levant farmers, and Jesus would have been quite familiar with it. (Evidently, hamburgers and bratwurst had not quite yet achieved popularity in the region, so the condiment wasn't widely used.) Many people now have small mustard seed pendants as reminders that even a mustard seed's worth of faith can move mountains (Matt 17:20/Luke 17:6) and the seeds are not rare as whole spice in American kitchens.
The problem is, the mustard seed isn't the smallest seed in the world. It wasn't necessarily even the smallest seed planted in the area ("Consider the lilies* of the field...")
Some apologists like to assert that Jesus didn't mean that the mustard seed was the smallest seed in the world, and they point to the phrase "which a man took and sowed in his field" as a qualifier that he meant locally. He doesn't really specify that though, and someone making a point to rural dwellers of an agricultural region would want to be a bit more specific to make his point.
Others point to translations that take the greek word mikronos and render it as "least of all seeds" rather than "smallest of all seeds." Thus they say Jesus was making a point about how the mustard seed was unassuming or meek, not about its physical size. However, the parable is about the size of Kingdom Come, not about its ostentation. Further, that usage of mikronos is fairly obscure and doesn't really appear anywhere else in the bible.
Now of course all this is "straining at gnats" in the end. But if someone is the Son of God, the creator of all things, shouldn't he have a little more familiarity with the local fauna? And if the Bible is inerrant, does that mean that the error was one that Jesus the otherwise perfect man made? Most modern believers won't be troubled by this hopefully. It's just fun to kick against the goads of those who insist on absolutism and perfectionism of the book.
*This is used only for dramatic/ironic purposes. Lily seeds actually vary a bit in size by variety but most are not quite as small as an average mustard seed. However, the Lotus flower has seeds so fine that they actually are perceived as a powder in volume. This flower is mentioned in Job twice and was sacred to and part of the creation myth of the ancient Egyptians, their country being one where Jesus is said to have spent some time. For him not to be familiar with the lotus is probably unreasonable, but it is possible he was simply of limited knowledge.