When I save out a JPEG version, for example to email for comment, I often name it by clicking the previous version of that JPEG to pick up the name. Then it's easy to forget to choose JPEG from the file type menu. The result is a PSD file with a .jpg suffix, which is never a good idea.
The best solution would be for Photoshop to obey the .jpg suffix and save it as a JPEG.
Other options include opening a dialog box to point out the inconsistency, or forcing the suffix to match the file type from the popup menu.