[Question]
  [
Your task is to write a program that makes your screen appear to have a bad pixel.
You should choose exactly one pixel on the screen, and exactly one channel between red, green and blue, and make its value either always 0 or always maximum (usually 255). The values of other channels should be the same as if your program didn't run.
You can choose the pixel and channel in whatever ways, such as hardcoding it or generate it randomly on each run. But it must always be visible on fairly modern hardwares.
If the color that originally should be displayed on that pixel has changed, your program should update in less than 0.5 seconds when the system is fast enough.
You cannot make assumptions about what is currently displayed on the screen (such as a taskbar using the default theme).
Your program doesn't need to work when a screensaver, login screen, effect from a window manager, etc, are active.
Shortest code wins.
  
[Answer]
# Bash on Linux - 25 bytes of Latin-1
*+3 from @wyldstallyns* /
*-2 from removing quotes* /
*-1 because I forgot how this works* /
*-9 from @Dennis*
```
printf ÿ>/dev/fb0;exec $0
```
Assumes that /dev/fb0 exists (it does on my Arch Linux system, and I think it should on any other Linux system). Requires root access as well. This does not work for me when X is running. On my system, this just constantly sets the blue channel of the top-left pixel to full (`ÿ` is 255).
[Answer]
# Visual C++, ~~102~~ ~~100~~ ~~99~~ 98 bytes
```
#include<Windows.h>
int main(){for(HDC d=GetDC(0);;Sleep(99))SetPixel(d,9,9,GetPixel(d,9,9)|255);}
```
Runs on Windows, directly using the Win32 API with Visual C++ compiler targeting the console subsystem. Uses the "screen" [device context](https://msdn.microsoft.com/en-us/library/windows/desktop/dd144871(v=vs.85).aspx) to set the red channel of the pixel at (9,9) to `0xFF`.
The sleep is necessary to allow other programs to draw in between the get/set - and 9ms was too short, leading to the pixel getting stuck1 on its initial colour.
---
1 Unfortunately, not quite the same type of stuck pixel this question is looking for...
[Answer]
# HolyC, 13 bytes
```
GrPlot(,9,9);
```
Puts a black dot on the persistent layer directly.
[![Layers](https://i.stack.imgur.com/WRK8c.png)](https://i.stack.imgur.com/WRK8c.png)
What it looks like.
[![Screenshot](https://i.stack.imgur.com/cDkA3.pngs)](https://i.stack.imgur.com/cDkA3.pngs)
[Answer]
# C#, ~~247~~ ~~244~~ ~~371~~ ~~366~~ ~~354~~ 352 bytes
Runs on Windows. Gets a [device context](https://msdn.microsoft.com/en-us/library/windows/desktop/dd144871(v=vs.85).aspx) for the entire screen, and repeatedly maximises the red channel at (9,9).
```
namespace System.Runtime.InteropServices{using I=IntPtr;class P{[DllImport("User32")]static extern I GetDC(I h);[DllImport("Gdi32")]static extern int GetPixel(I h,int x,int y);[DllImport("Gdi32")]static extern int SetPixel(I h,int x,int y,int c);static void Main(){for(I d=GetDC((I)0);;Threading.Thread.Sleep(99))SetPixel(d,9,9,GetPixel(d,9,9)|255);}}}
```
I originally didn't want to import `GetPixel`/`SetPixel`, but there's no particularly easy way to read a single pixel off a `Graphic`. So right now this is effectively the same as [my VC++ attempt](https://codegolf.stackexchange.com/a/102961/4163). Maybe saving to bitmap will be shorter...
---
-5 bytes thanks to @TuukkaX
[Answer]
# SmileBASIC, 20 bytes
```
SPSET.,299,99,1,1,33
```
Updates constantly.  
Sets sprite `0` to a `1`x`1` area at `299`,`99` on the sprite sheet (which is a red pixel).  
`33` is the display attribute, which is `1` (display on) + `32` (additive blending).
[Answer]
# Java 7, 266 bytes
```
import java.awt.*;public class K extends java.applet.Applet{public static void main(String[]a){new K();}Label l=new Label(".");public K(){add(l);}public void paint(Graphics g){s(Color.red);s(Color.green);s(Color.blue);repaint();}void s(Color c){l.setForeground(c);}}
```
I ran this on windows 7. Opens a Java Applet which has a white background by default. Adds a label with a period and then changes the color of the label ad nauseum. 
[Answer]
# Tcl/Tk, 61
```
wm at . -tr #F0F0F0
wm o . 1
grid [canvas .c]
.c cr o 9 9 9 9
```
On the image there is a black pixel near the upper left corner of the Vivaldi icon:
[![badpixel](https://i.stack.imgur.com/owJWt.png)](https://i.stack.imgur.com/owJWt.png)
---
If on an interactive shell, there are abbreviations of commands available:
# Tcl/Tk, 57
```
wm at . -tr #F0F0F0
wm o . 1
gri [can .c]
.c cr o 9 9 9 9
```
There is a black pixel over the V white area of the Vivaldi icon:
[![badpixel](https://i.stack.imgur.com/iJwSM.png)](https://i.stack.imgur.com/iJwSM.png)
]