The ~50 ppm peak is most likely the carbon next to the aldehyde C=O. The exact chemical shift of a carbon depends on the electron density that sits on it. Because this carbon is next to the aldehyde, which is electron withdrawing, it appears at a slightly higher chemical shift than your average alkyl carbon. It's a bit of a tricky one considering that it sits in the correct ppm range for C-O, but you should always remember that those ranges are just guidelines, and that other groups can give rise to those same chemical shifts.