If you don't store it in sealed containers, they can draw a pantry weevil infestation (as can most dried goods, including cereals, rice, and pasta). They aren't inherently toxic and only a full-on infestation will cause spoilage, but they're obviously not palatable in any quantity.

You can prevent them by simply putting things like flour bags in air-tight sealable containers. If you catch some weevils in a bag of flour before it turns into an infestation, you can freeze the flour for four to six days to kill any eggs and larvae and sift out the remains.

If you don't mind the very slight possibility of flavor contamination, you can also store garlic cloves or bay leaves openly near your flour, which naturally repel weevils.