user77036: TOF050C-VL6180X Sensor was not responding, so I checked it with a multi-meter, and the VIN, GND solder pins on it read 4.7V. Double-checked Mega voltage, without anything connected, and it shows stable 3.6V. Connected a different ToF and it works perfectly, with a 3.6V on it.
So that proves that sensor is damaged (might've powered it with 5V at first), but I don't understand how is it taking more than the regulated 3.3V? If it was shorted, it would read 0V, and if something burnt, it would be open circuit and show ~3.3V, right? So how is it just getting 4.7V out of nowhere? Any ideas on what I'm not understanding here?
Thank you!
EDIT: More info, tried to isolate issue as much as I could
1) The non-working sensor shows 4.7V

2) The working sensor shows 3.4V and outputs distance to serial

3) This is where it gets weird. I uploaded the simple blink sketch and now the working sensor shows 4.7V too

4) So I tried disconnecting the SDA/SCL, while leaving power connected, with the blink sketch running, and it's back to 3.3V!

**Tried the same steps with a different Mega, and it always shows 3.3V, no matter the configuration!
Now I'm very lost. So is the Arduino semi-damaged, with the I2C communication acting weird? Is it still safe to use with the project, if it technically works?

user70020: The Arduino Wire enables the internal pullup resistor on SDA and SDL. The internal pull-ups are specified at 20k to 50k. So you are providing 5V (or there about) to the sensor, though through a high resistance connection. So, the voltage isn't coming from nowhere.
It is not unlikely that there protection/clamp diodes in the sensor IC or on the sensor board that connect these signals forward through the diode to the IC's 3.3v connection.
So your SCL/SDA can be seen as very weakly (through roughly 35k) trying to backfeed your 3.3V regulator which is probably just blocking the current. If the sensor/board is using no current to speak of, either because it just doesn't normally or because it's dead, you would measure 5V minus a diode drop, or roughly the 4.7V you're seeing.
If you assume the internal resistors are both at 20K as a "worst" case scenario for a calculation, the current needed through one to drop 5v to 3.3v is 85uA. So with 170uA load on the board that would drop the 3.3V pin  connection back to its 3.3V level. About 170uA will flow through a 20k resistor with 3.3V across it. So if you want experiment take any resistor from about 470ohms all the way to just shy of 20k in parallel with your 3.3V / GND measurement and see if your 4.7V measurement doesn't drop right down to the normal 3.3V output.

77036: <p>TOF050C-VL6180X Sensor was not responding, so I checked it with a multi-meter, and the VIN, GND solder pins on it read 4.7V. Double-checked Mega voltage, without anything connected, and it shows stable 3.6V. Connected a different ToF and it works perfectly, with a 3.6V on it.</p>
<p>So that proves that sensor is damaged (might've powered it with 5V at first), but I don't understand how is it taking more than the regulated 3.3V? If it was shorted, it would read 0V, and if something burnt, it would be open circuit and show ~3.3V, right? So how is it just getting 4.7V out of nowhere? Any ideas on what I'm not understanding here?</p>
<p>Thank you!</p>
<p><strong>EDIT:</strong> More info, tried to isolate issue as much as I could</p>
<p><strong>1)</strong> The non-working sensor shows 4.7V
<a href="https://i.stack.imgur.com/DXhSZl.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/DXhSZl.jpg" alt="1) The non-working sensor shows 4.7V on multi-meter" /></a></p>
<p><strong>2)</strong> The working sensor shows 3.4V and outputs distance to serial
<a href="https://i.stack.imgur.com/qzoAHm.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/qzoAHm.jpg" alt="Working sensor shows 3.4V on multi-meter" /></a></p>
<p><strong>3)</strong> This is where it gets weird. I uploaded the simple blink sketch and now the working sensor shows 4.7V too</p>
<p><a href="https://i.stack.imgur.com/JpmM5m.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/JpmM5m.jpg" alt="Working sensor shows 4.7V on multi-meter" /></a></p>
<p><strong>4)</strong> So I tried <strong>disconnecting the SDA/SCL</strong>, while leaving power connected, with the blink sketch running, and it's back to 3.3V!</p>
<p><a href="https://i.stack.imgur.com/kwlBbm.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/kwlBbm.jpg" alt="Arduino outputs 3.3V to sensor" /></a></p>
<p>**Tried the same steps with a different Mega, and <strong>it always shows 3.3V, no matter the configuration!</strong></p>
<p>Now I'm very lost. So is the Arduino semi-damaged, with the I2C communication acting weird? Is it still safe to use with the project, if it <em>technically</em> works?</p>
 70020: <p>The Arduino Wire <a href="https://github.com/arduino/ArduinoCore-avr/blob/1.8.5/libraries/Wire/src/utility/twi.c#L87-L89" rel="nofollow noreferrer">enables the internal pullup resistor</a> on SDA and SDL. The internal pull-ups are specified at 20k to 50k. So you <em>are</em> providing 5V (or there about) to the sensor, though through a high resistance connection. So, the voltage isn't coming from nowhere.</p>
<p>It is not unlikely that there protection/clamp diodes in the sensor IC or on the sensor board that connect these signals forward through the diode to the IC's 3.3v connection.</p>
<p>So your SCL/SDA can be seen as very weakly (through roughly 35k) trying to backfeed your 3.3V regulator which is probably just blocking the current. If the sensor/board is using no current to speak of, either because it just doesn't normally or because it's dead, you would measure 5V minus a diode drop, or roughly the 4.7V you're seeing.</p>
<p>If you assume the internal resistors are both at 20K as a &quot;worst&quot; case scenario for a calculation, the current needed through one to drop 5v to 3.3v is 85uA. So with 170uA load on the board that would drop the 3.3V pin  connection back to its 3.3V level. About 170uA will flow through a 20k resistor with 3.3V across it. So if you want experiment take any resistor from about 470ohms all the way to just shy of 20k in parallel with your 3.3V / GND measurement and see if your 4.7V measurement doesn't drop right down to the normal 3.3V output.</p>
