The front-facing camera seems to be liable to moving within its housing, which results in a grey crescent shape appearing on the right side of the camera.
You can see the problem in the photo above. There's a barely perceptible grey area showing through where the camera seems to have shifted slightly to the left.
People all over the internet are experiencing the same problem.
A Reddit thread titled "Is your iPhone 6 front-facing camera misaligned?" has more than 150 comments. The replies are almost all variations on "Mine has the exact same issue" and "Same." Currently, almost none of the users complaining about it say it actually impacts the photos being taken. In other words, if the thing still works there is no need to replace it - it's just cosmetic.
There's also a Mac Rumors thread about the problem. Here's a photo posted by "bhaffar" that shows the same issue on his iPhone 6:
And here's another photo of the camera alignment problem, this time posted by Mac Rumors forum user "Oridus":
Customers are also complaining about the problem on iFixit. Here's a photo posted by "calvin" that shows his camera has also moved to the left:
Apple seems to be aware of the camera misalignment problem. Customers are reporting that the company is offering on-the-spot replacements to people who have presented their phones in Apple Stores and pointed out that they're suffering from the grey crescent.
Reddit user spykeman1528, who claims to work at an Apple Store, says that Apple hasn't yet figured out what is causing the problem:
Apple Store employee here, we don't know why they shift and if you choose we can replace the display which will replace the front facing camera or just swap the device all together. I usually just offer the full unit swap since customers feel the entire device is defective at that point. I only recommend display repairs if a customers data is not backed up and that poses less of a risk for data loss than obvious full data loss of a whole unit swap if the customer is not backs up.
We reached out to Apple for comment on this story and will update this post if we hear back.