On Android phones you can activate a feature called pattern lock. This feature can be compared to a Windows computer asking for password after the screen saver kicked in, only instead of a password you use a pattern. You get a screen with 9 dots and you draw a pattern of your choice with 4 of them. If you forget your pattern (or have kids that likes to play with phones) you will after 3 faulty attempts be unable to use the pattern to unlock it. Instead you are asked for your Google account and password (that is setup with the phone of course). This is where the problem starts.

For many, many users providing the username and password won’t change a thing. For some people changing the password via a computer has worked but that is uncommon. Most people ends up with a very expensive paperweight. Have no fear, there are solutions to almost everything!

Update

End Update

In this case the solution is to make a factory reset (sometimes referred to as hard reset or wipe). This will erase all data on your phone (not on your SD-card). Your contacts and mails should be in sync with the Google servers, no problem there then. Downloaded applications needs to be installed again but there is a list of your previous downloads in the Android Market->My Downloads. Other things though are lost if you haven’t backed them up manually with a 3-rd party application. Example on things you will lose: Call logs, text messages, application settings.

This is your 3 step solution

1. Factory reset your phone (links to Android device manuals can be found below, it differs between devices)
2. Setup your phone just like you did the first time you unpacked it (use the same Google Account if you want your old contacts back!)
3. Restore any manual back-ups and install your previous applications from the My Downloads list.

How do I factory reset? HTC leads the way and put this information in their manuals, other manufacturers should learn from this!

HTC Hero manual:
http://member.america.htc.com/download/web_materials/Manual/HTC_Hero/091201_Hero_HTC_English_UM.pdf

HTC Magic (a.k.a. My Touch) manual:
http://member.america.htc.com/download/web_materials/Manual/HTC_Magic/090507-HTC-Sapphire-UM.pdf

HTC Tattoo manual:
http://member.america.htc.com/download/web_materials/Manual/HTC_Tattoo/091009_HTC_Tattoo_English_UM.pdf

HTC ERIS manual:
http://member.america.htc.com/download/Web_materials/Manual/DROID_ERIS_Verizon/DROID_ERIS_Verizon_English_UM_11_5.pdf or:

1. With the phone turned off, PRESS and HOLD Volume down and Send buttons CALL buttons (the manual says SEND and with that they obviously mean Call)
2. Press the END/POWER button. You will see a screen with three Androids at the bottom of your screen
3. Follow on screen instructions

Samsung Galaxy (note old Galaxy i7500, Galaxy S further down):
1. Turn off your Galaxy.
2. Hold volume_down+call+power button.
3. After boot, press menu button to start recovering.

Motorola Droid (a.k.a. Milestone): 1. Turn the Droid off. If it won’t turn off, take the battery out then place it back in.
2. Open the keyboard and press the X key while pressing and holding Power. The device will power on. Keep holding the buttons down until a yellow triangle appers.

3. Press the Camera and Volume Up buttons at the same time to get to the menu.
4. Use the direction pad to the right of the keyboard to select the Format option.

Nexus One:
1. Turn the phone off. If it’s frozen in the on state, pull the battery.
2. Press and hold both the Home and Back buttons simultaneously.
3. While continuing to hold the buttons, press the Power button.
4. Release Home and Back.
5. Follow the instructions on the screen to complete the hard reset.

T-Mobile Pulse
If you want to do a Hard Reset but can’t do it through the menu, please use the update from T-Mobile UK website: T-Mobile – Help & Support

But use the file from the “Android updates” section and the following piece:
November 2009: T-Mobile Pulse software

This software update should be used ONLY when your T-Mobile Pulse has been security locked and you do not have a valid password.

1. If you right-click on your mouse here and select the “Save target as..” option – it should take about 2 minutes to download depending on your internet connection speed.
   PLEASE NOTE: Installing this update will remove all existing data on your T-Mobile Pulse.
2. Then in your pc format the SD card. then copy the folder dload to your new formatted SD card. So the folder with the two files will be on there.
3. Insert your SD card in the phone
4. Then when your phone is off, do the following:
   Vol + key & “hang up button” – keep them both together pushed and then turn on the phone. Then the update will start automatically. Once the update is finished it might hang to long in the “phone will restart automatically”. If so, take out the battery again and turn the phone on normally. then it all will go as normal.
5. Once the update is done, you will have to configure your phone again, which also means you can choose the language again.

T-Mobile Pulse instructions from “inge75″ in the Android forums.

Sony Ericsson X10 -mini, pro etcetera

1. Go to: www.sonyericsson.com
2. Click the support tab and then Download Software
3. Download the Update Service application
4. Run the application as administrator, choose your model and follow the instructions.

Sprint (Samsung) Epic 4G and Samsung Galaxy S

1. With your phone turned off. Pull the battery out and reinsert it if needed.
2. Press and Hold “Volume Down” button
3. Press and Release the “Power” button (while still holding “Volume Down”
4. Your phone should now boot to a menu where you can choose Fastboot, Recovery, Clear Storage, or Simlock
5. Select Clear Storage by pressing the “Volume Down” button
6. Press the “Power” button
7. Confirm your decision with “Volume up” (Cancel with “Volume down”)

Motorola (Nextel) i1 – Now confirmed, working!

1. Make sure your device is turned off, pull the battery if needed
2. Push and hold: “Volume Up” + “Volume Down” + “Call” + “Center button”, press “Power”

This is information I gathered from both here and there, manuals and end users. The usual suspects: Swedroid, Android forums, xda-developers and knowyourcell