C'mon man... Learn genetics. Seriously. Recessive genes can ABSOLUTELY express themselves in a child from even one parent being a carrier regardless of how the parents' genes express. In addition, random mutations can and DO produce weird things.
Yeah, it's entirely possible for a child of two brown-haired parents to have blonde or even red hair.
For one thing, blonde hair tends to get darker as you get older, and so a lot of people who were born with blonde hair can end up with dirty blonde, light brown or even dark brown hair. This can mask the fact that they technically have the genetics for blonde hair.
Another factor is that blonde hair is a recessive gene, which (if I remember my genetics correctly) means that not everyone who has the gene will actually gets blonde hair because they need two copies of it instead of one; something which can get further confused by whether you're born with XX, XY, or some other combination of chromosomes (I think, if I remember correctly, that some genes are more likely to express themselves with XY chromosomes or vice versa, not sure if that's true for hair color). So you can have a family with brown hair, but if the genetics line up just right, you can get a blonde-haired kid (if I understand correctly, this is also true for red hair).
Yeah, I was light brown and blonde as a kid, black now, I have one son with blonde hair, one with black, My dad commented once that my blonde son looks just like I did as a kid, and my black haired son looks just like I do now. Also two blonde daughters, the oldest in her teens, her hair is darker in spots, but still some blonde. My youngest had a lot of red mixed in as well, but that's all gone by now.
In fact a similar thing happened in my family but my parents have darkening hair, and my siblings have minimally darkening blonde and brown hair. There is even some red mixed in somewhere.