robarnold is right about Safe Mode and the article he posted (which is now linked in my quoted reply) provides more information about Safe Mode. Please see the attached picture to see what Safe Mode looks like on our phones. If your phone were to enter Safe Mode, those words would appear on all your screens. As per the article, most if not all your apps will be suspended or not work.
Basically there are two known ways to enter Safe Mode:
1. Android detects a problem with a newly installed app and force-closes it while entering into Safe Mode.
2. A combination of key presses at power application;
Newly installed app force-closes the system
The gist of the article provided by Rob says that if a newly installed app causes system conflicts, Android will force close that app to prevent it from possibly doing malicious harm. At the same time, Android will suspend the majority of all other apps and display "Safe mode" in the lower left corner of your phone as per the picture.
The reason for this is to provide the user an opportunity to clean the system of the app which is causing the force closure. Chances are it the most recent app installed but it could be a combination of apps that are coming into conflict with one another.
If you suspect a newly installed app of causing this problem and you see Safe Mode on screen, STOP, COLLECT YOUR WITS and BREATHE NORMALLY. You have several options.
SAFE MODE OPTIONS
1. Press the System or Menu Button in the lower left corner and navigate to Manage apps.
2. Find the most recent app you installed.
3. Force close it.
4. Disable it if possible.
5. Remove this app.
6. Reboot the boot to see if the Safe mode is gone.
If the Safe mode label is still present, the article suggests that it may be necessary to press several keys to achieve the removal of the Safe mode label and its effect of suspending the other apps. In essence, we now move to the second discussion of entry into Safe mode:
1. Safe mode can be entered by pressing buttons at start-up. Most likely, this is what I did today as I was getting ready to leave for an appointment. The Safe Mode label freaked me out and when I could use the Rebooter app or any other app, I became scared.
2. I don't know exactly which buttons I pressed but it mostly likely was the System and Home (center big button) and maybe even the Escape/Back button on the right. My phone slipped slightly as I turned it on and my left thumb was flying as I manuevered my left hand to balance the phone.
3. The right combination of buttons, which supposedly are the same to enter Safe Mode, are the same to exit Safe Mode.
4. If you are certain you entered Safe Mode by accident - meaning you know you did not recently or just now installed an app that force closed Android, then you just have to find the combo to exit it.
5. The article suggests that pressing the volume buttons might clear the Safe Mode on the first reboot after initial entry into Safe Mode.
6. If the Safe Mode label disappears after pressing the volume buttons, your phone is okay.
7. If the label is still present, try rebooting the phone again.
The article suggests as per my experience that if you cannot clear the Safe Mode, your last resort is to do a factory reset of the phone. Fortunately in my case, I had ClockworkMod Recovery available and restored my system via a backup recovery.
ronbo's Final Thoughts
Hope I never see Safe Mode again (and, I won't go looking for it) but I recommend you learn how to do a Factory Reset by pressing and holding the Volume Down Button with a simultaneous press of the Power Button. You want to see the Factory Test and Reset Menu when you are calm. TRUST ME ON THAT!
Yes safe mode exists and is a useful if not essential tool for those of who like to experiment with their Android devices.
I'd suggest that it will always be worthwhile to know how to access Safe Mode and recover from it if you are going to install unsigned apps.
The following link may more useful to those who would like to know more: