P: JPEG format tripping up other programs
Lightroom CC writes JPEGs in an atypical layout that trips up other programs. While they appear to be strictly conforming to industry standards, the unusual layout has caused problems for at least two users: one who had problems uploading photos to real-estate services, and another whose own software tripped over the layout.
LR writes the APP1 header as: TIFF header, 114 unused bytes, ExifIFD, IFD1, IFD0. Whereas Photoshop CC uses a more traditional layout: TIFF header, no unused bytes, IFD0, ExifIFD, IFD1. Here's an Exiftool dump of the beginning of a LR JPEG: 
And here's a dump of the beginning of a Photoshop JPEG: 
I've tested LR JPEGs with 12 Mac and Windows programs and 2 online services, all of which read them just fine:
Mac: Preview, Photoshop CC 2014, ColorSync Utility, Firefox, Chrome, Lattice, Paintbrush, Safari, Word
Windows 8.1: File Explorer Preview, Irfanview, Paint, Windows Photo Viewer
Online services: Flickr, Zenfolio
Even though LR's JPEGs may be strictly conforming, if it wrote the JPEGs without the unused header bytes and it put IFD0 immediately after the TIFF header, as Photoshop does, then there would be fewer problems with other programs choking on LR's JPEGs.
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