Missense errors in translation occur at rates of one per 103-104 codons; at an error rate of 5 × 10-4, 18% of proteins expressed from an average length (400 codon) gene contain at least one missense substitution.

— Mistranslation-Induced Protein Misfolding as a Dominant Constraint on Coding-Sequence Evolution. D. Allan Drummond and Claus O. Wilke. Cell 134:341-352 (25 July 2008) doi:10.1016/j.cell.2008.05.042

(My emphasis.)

Further information: Jon Yewdell’s “DRiPs” concept (RNA, protein, and information)