I just watched a gif of Let it Go with the hair flip and you're right. I guess I never noticed it before because it happens pretty fast in the movie and I wasn't really looking for it.
You mean the thing that was fixed?
for reference heres the original:
Want to know the source of the "mistake?" The promotional music video they released on youtube for the song. The feature didn't have this in it
Not trying to come off as an "apologist" in any way, but a lot of time and effort goes into making CG movies, and a lot of the times deadlines get pushed up for marketing purposes. This also happens with regular movies too, thats why a lot of times there are scenes in the trailer that are not in the movie (or they are, but different).
RE: how to train your dragon 2 and its "new software". A little off point...it was a new VERSION of the software that was developed in the 80s
It was re-written to meet todays hardware, but overall the system works the same as always. Similar to how Pixar has Marionette and Renderman, a lot of studios build in-house software to work with their pipeline. Each company's respective suite has something it does better than somebody elses. I do always find it amusing with every CG release, every studio has to boast about something they "never were able to do before they developed new software". For Pixar, it was hair and water physics. For Disney it was hair, clothes, and snow. For dreamworks its usually fire and particles.
Now..and sorry for babbling on about this..yet again...
There is always a huge downside to using "custom" software. Is it easy to use? sure it is......once you are trained in it. No college/university teaches these programs. Upon getting hired by a bigger studio, you have a very limited time to get familiarized with how their system works. While you are learning, you are not being productive..and thats not something they like. Its because of this that Disney/Pixar have actually started adapting their software to "plug in" directly into the API's of other, commercially available software. Namely Maya. This is widely taught by all universities and by just having it so you can work within it while harnessing the power of their development advances, you eliminate the learning curve for new hires.
In the end does ANY of it matter? Not in the least. Sure the new tools are neat..but all it does is make it all EASIER. Few of what you see today on screen was impossible 10 years ago, it just took more work.