Precipitates appear in serum for a variety of reasons:
Repeated freeze-thaw cycles denature serum lipoproteins and cause turbidity. This can be minimized by preparing single-use aliquots.
Serum is processed rapidly at cold temperatures to retain growth properties, which may leave some fibrinogen in serum, which may convert to fibrin on thawing. The excess amount of fibrin in serum is visible as flocculent material, which can be removed by centrifugation at 400xg.  Filtering serum to remove flocculent is not recommended, as filters may become clogged, or unintentional retention of desirable proteins on the filter may occur.