Holy cow, I wrote a book!
Keytips are those little pop-up keyboard accelerator thingies
that appear on the Ribbon when you tap the Alt key:
46 00 0C 20 46 00 0C 20
----- ----- ----- -----
F ????? F ?????
The question marks are U+200C,
formally known as
ZERO WIDTH NON-JOINER.
discussed the character (and its evil twin the
ZERO WIDTH JOINER) some time ago.
The ZERO WIDTH NON-JOINER (or ZWNJ to his friends)
is a hint to the font engine that
the characters on opposite sides of the ZWNJ should not be combined
into a ligature.
In English, the ZWNJ would prevent two consecutive lowercase "f"s
from being converted into a "ﬀ" ligature.
Ligatures are fading from use in contemporary printing,
probably due to the rise of computers. Back in the old days,
you saw all sorts of neat ligatures, like "ﬆ".
Breaking up the ligature is important when presenting keyboard
Imagine if the keyboard accelerator for a key sequence was "A"
followed by "E".
If this were displayed as "Æ", users would waste their
time looking for an
"Æ" key on their keyboard.
Although English doesn't have many ligatures any more,
many other languages
still employ them heavily.
(You may have noticed that the keytip was a bit overzealous with
the ZWNJ, putting one at the end of the string even though there
was nothing for the second F to be unjoined from!)
So if you encounter one of these ZWNJ characters, don't be afraid.
He's just there to break things up.
And as Michael notes,
ZWNJ and ZWJ "are supposed to be ignored in things like
the Unicode Collation Algortihm."