What are a sundial flaws?

Flaws is the plural for flaw, a hidden or not obvious defect in something or someone.

A sundial is a device that, when properly oriented, can indicate the time of day based on the direction sunlight is coming from at the time. More information can be found at the Related Link.

Sundials tell the time by the shadows cast by the sun onto a circular calibrated surface, much like a clock but without mechanism or chips.

A sundial is pretty much an analogue clock, except instead of hands, it uses the suns shadows to point at hour lines on its base.

If correctly aligned it'll tell the time by casting a shadow from the sun over a marked face.