<s>
An	O
undocumented	B-General_Concept
feature	I-General_Concept
is	O
an	O
unintended	O
or	O
undocumented	O
hardware	B-Architecture
operation	O
,	O
for	O
example	O
an	O
undocumented	B-Language
instruction	I-Language
,	O
or	O
software	B-Architecture
feature	I-Architecture
found	O
in	O
computer	B-Architecture
hardware	I-Architecture
and	O
software	O
that	O
is	O
considered	O
beneficial	O
or	O
useful	O
.	O
</s>
<s>
Sometimes	O
the	O
documentation	O
is	O
omitted	O
through	O
oversight	O
,	O
but	O
undocumented	B-General_Concept
features	I-General_Concept
are	O
sometimes	O
not	O
intended	O
for	O
use	O
by	O
end	O
users	O
,	O
but	O
left	O
available	O
for	O
use	O
by	O
the	O
vendor	O
for	O
software	O
support	O
and	O
development	O
.	O
</s>
<s>
Also	O
,	O
some	O
unintended	O
operation	O
of	O
hardware	B-Architecture
or	O
software	O
that	O
ends	O
up	O
being	O
of	O
utility	O
to	O
users	O
is	O
simply	O
a	O
bug	B-Error_Name
,	O
flaw	O
or	O
quirk	O
.	O
</s>
<s>
Since	O
the	O
suppliers	O
of	O
the	O
software	O
usually	O
consider	O
the	O
software	O
documentation	O
to	O
constitute	O
a	O
contract	O
for	O
the	O
behavior	O
of	O
the	O
software	O
,	O
undocumented	B-General_Concept
features	I-General_Concept
are	O
generally	O
left	O
unsupported	O
and	O
may	O
be	O
removed	O
or	O
changed	O
at	O
will	O
and	O
without	O
notice	O
to	O
the	O
users	O
.	O
</s>
<s>
Some	O
user-reported	O
defects	O
are	O
viewed	O
by	O
software	B-Application
developers	I-Application
as	O
working	O
as	O
expected	O
,	O
leading	O
to	O
the	O
catchphrase	O
"	O
it	O
's	O
not	O
a	O
bug	B-Error_Name
,	O
it	O
's	O
a	O
feature	O
"	O
(	O
INABIAF	O
)	O
and	O
its	O
variations	O
.	O
</s>
<s>
Undocumented	B-Language
instructions	I-Language
,	O
known	O
as	O
illegal	B-Language
opcodes	I-Language
,	O
on	O
the	O
MOS	B-General_Concept
Technology	I-General_Concept
6502	I-General_Concept
and	O
its	O
variants	O
are	O
sometimes	O
used	O
by	O
programmers.These	O
were	O
removed	O
in	O
the	O
WDC	B-General_Concept
65C02	I-General_Concept
.	O
</s>
<s>
Video	O
game	O
and	O
demoscene	O
programmers	B-Application
for	O
the	O
Amiga	O
have	O
taken	O
advantage	O
of	O
the	O
unintended	O
operation	O
of	O
its	O
coprocessors	B-General_Concept
to	O
produce	O
new	O
effects	O
or	O
optimizations	O
.	O
</s>
<s>
In	O
2019	O
,	O
researchers	O
discovered	O
that	O
a	O
manufacturer	O
debugging	O
mode	O
,	O
known	O
as	O
VISA	O
,	O
had	O
an	O
undocumented	B-General_Concept
feature	I-General_Concept
on	O
Intel	O
Platform	O
Controller	O
Hubs	O
,	O
chipsets	O
included	O
on	O
most	O
Intel-based	O
motherboards	O
,	O
which	O
makes	O
the	O
mode	O
accessible	O
with	O
a	O
normal	O
motherboard	O
.	O
</s>
<s>
Since	O
the	O
chipset	O
has	O
direct	B-General_Concept
memory	I-General_Concept
access	I-General_Concept
this	O
is	O
problematic	O
for	O
security	O
reasons	O
.	O
</s>
<s>
Undocumented	B-General_Concept
features	I-General_Concept
(	O
for	O
example	O
,	O
the	O
ability	O
to	O
change	O
the	O
switch	O
character	O
in	O
MS-DOS	B-Application
,	O
usually	O
to	O
a	O
hyphen	O
)	O
can	O
be	O
included	O
for	O
compatibility	B-Device
purposes	O
(	O
in	O
this	O
case	O
with	O
Unix	B-Application
utilities	O
)	O
or	O
for	O
future-expansion	O
reasons	O
.	O
</s>
<s>
In	O
some	O
cases	O
,	O
software	B-Error_Name
bugs	I-Error_Name
are	O
referred	O
to	O
by	O
developers	B-Application
either	O
jokingly	O
or	O
conveniently	O
as	O
undocumented	B-General_Concept
features	I-General_Concept
.	O
</s>
<s>
This	O
usage	O
may	O
have	O
been	O
popularised	O
in	O
some	O
of	O
Microsoft	O
's	O
responses	O
to	O
bug	B-Error_Name
reports	O
for	O
its	O
first	O
Word	B-Application
for	I-Application
Windows	I-Application
product	O
,	O
but	O
does	O
n't	O
originate	O
there	O
.	O
</s>
<s>
The	O
oldest	O
surviving	O
reference	O
on	O
Usenet	B-Application
dates	O
to	O
5	O
March	O
1984	O
.	O
</s>
<s>
Between	O
1969	O
and	O
1972	O
,	O
Sandy	O
Mathes	O
,	O
a	O
systems	O
programmer	B-Application
for	O
PDP-8	B-Device
software	O
at	O
Digital	O
Equipment	O
Corporation	O
(	O
DEC	O
)	O
in	O
Maynard	O
,	O
MA	O
,	O
used	O
the	O
terms	O
"	O
bug	B-Error_Name
"	O
and	O
"	O
feature	O
"	O
in	O
her	O
reporting	O
of	O
test	O
results	O
to	O
distinguish	O
between	O
undocumented	O
actions	O
of	O
delivered	O
software	O
products	O
that	O
were	O
unacceptable	O
and	O
tolerable	O
,	O
respectively	O
.	O
</s>
<s>
Undocumented	B-General_Concept
features	I-General_Concept
themselves	O
have	O
become	O
a	O
major	O
feature	O
of	O
computer	O
games	O
.	O
</s>
<s>
Developers	B-Application
often	O
include	O
various	O
cheats	O
and	O
other	O
special	O
features	O
(	O
"	O
easter	O
eggs	O
"	O
)	O
that	O
are	O
not	O
explained	O
in	O
the	O
packaged	O
material	O
,	O
but	O
have	O
become	O
part	O
of	O
the	O
"	O
buzz	O
"	O
about	O
the	O
game	O
on	O
the	O
Internet	O
and	O
among	O
gamers	O
.	O
</s>
<s>
The	O
undocumented	B-General_Concept
features	I-General_Concept
of	O
foreign	O
games	O
are	O
often	O
elements	O
that	O
were	O
not	O
localized	O
from	O
their	O
native	O
language	O
.	O
</s>
<s>
Closed	B-Application
source	I-Application
APIs	B-Application
can	O
also	O
have	O
undocumented	O
functions	O
that	O
are	O
not	O
generally	O
known	O
.	O
</s>
