When a soft-hyphen (­) is preceded by another soft-hyphen and one intervening character, the hyphen and intervening character are dropped when the word is wrapped at lines end.
The source code shows the full word without the dropped letter, so this appears to be a display issue only - but a very inconvenient one for us who use hyphenation on our websites.
This has been confirmed on Mac and Windows in Safari and Chrome.
I can reproduce this with shipping Safari, but not with a nightly build.
This was already fixed as <rdar://problem/5607547>, and is tested with fast/text/soft-hyphen-2.html.