I will give you the classical and non-classical answer.
The classical answer is that there is very little risk from over-consuming folate or taking a single very large dose, although a single large dose might cause some GI tract upset. Some research shows it might speed up growth of already existing tumors. The one real concern is that if you are also vitamin B-12 deficient, consuming a lot of folate can hide B-12 deficiency symptoms: meaning B-12 deficiency is still doing damage without you knowing.
The non-classical answer is that high intake of folate, more specifically, in folic acid form (fortified foods and supplements) might be contributing to chronic disease in a much more non-specific way that takes much longer to manifest. The immune system and brain are the two organs usually discussed to be at risk here. Unfortunately, we really don't have a good idea of what is going on here or a mechanism to describe it.