user31742: I have a LCD connected to a Arduino Mega 2560 using the instructions in LiquidCrystal library. It is not the first time I am using a 16x2 LCD but I never have encountered a problem like this. When I first connect the Arduino Mega to PC I see a row of blocks in first row of LCD which looks like normal to me. Then after that there is a block of white under each character. 
Now I have searched a little and saw whole blocks which was due to bad soldering or bad wiring or etc. I have checked soldering and wiring twice. used 3 different LCD's and 2 different Mega's and it is still the same.
Any suggestions?
These are the images from when I turn it on:
 
[SOLVED] Update :
Looks like the POT was broken and did not respond the way it used to. So i replaced V0 with a 2.5k resistor and connected them to GND.
Here is the updated image :
 

user40731: If your contrast is not adjustable by a pot, there's a resistor that goes on the contrast pin that controls the level of contrast, which is what is causing the dark blocks.

user4143: The row of white blocks is what you get before the screen's initialized. It's normal.
The shadow behind the characters is because you have the contrast turned up too high. Adjust it down a little.

31742: <p>I have a LCD connected to a Arduino Mega 2560 using the instructions in <code>LiquidCrystal</code> library. It is not the first time I am using a 16x2 LCD but I never have encountered a problem like this. When I first connect the Arduino Mega to PC I see a row of blocks in first row of LCD which looks like normal to me. Then after that there is a block of white under each character. </p>

<p>Now I have searched a little and saw whole blocks which was due to bad soldering or bad wiring or etc. I have checked soldering and wiring twice. used 3 different LCD's and 2 different Mega's and it is still the same.</p>

<p>Any suggestions?</p>

<p>These are the images from when I turn it on:</p>

<p><a href="https://i.stack.imgur.com/B4eOUm.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/B4eOUm.jpg" alt="image1"></a> <a href="https://i.stack.imgur.com/kOyOEm.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/kOyOEm.jpg" alt="image2"></a></p>

<p><strong>[SOLVED] Update :</strong></p>

<p>Looks like the POT was broken and did not respond the way it used to. So i replaced <code>V0</code> with a 2.5k resistor and connected them to GND.
Here is the updated image :
 <a href="https://i.stack.imgur.com/YluSa.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/YluSa.jpg" alt="image"></a></p>
 40731: <p>If your contrast is not adjustable by a pot, there's a resistor that goes on the contrast pin that controls the level of contrast, which is what is causing the dark blocks.</p>
 4143: <p>The row of white blocks is what you get before the screen's initialized. It's normal.</p>

<p>The shadow behind the characters is because you have the contrast turned up too high. Adjust it down a little.</p>
