Quick post: I was connecting a sensor to my Arduino Uno R3 and when I was going to ground the sensor I accidentally put it into the 5V pin on the Uno. TO CLARIFY, the sensor GND went to the Uno 5V momentarily. I have run several sketches on the Arduino after and they all work, including with the sensor mentioned prior. My main concern is that after I power on the Arduino from USB, the L LED begins to slowly get brighter until it reaches its maximum brightness. Based on what I have described, could I have caused damage to the board itself? Or am I overreacting?
So this occurrence of the LED fading in happened only the first time after you shorted the "5V" pin?
On a real UNO, the "L" LED - controlled by but not directly connected to pin 13 - randomly goes on and off if pin 13 is neither connected or defined as anything other than INPUT.
This sounds like the polyfuse resetting after it overheated. Generally, you should wait an hour or two after tripping the polyfuse. 
dominicj:
could I have caused damage to the board itself? Or am I overreacting?
It is possible to have caused damage, or stressed out a component or components. But if the application isn't life-threatening or puts somebody or anything at risk of injury, or property at risk of damage etc ......... then don't worry about this particular incident too much. But just learn from it ------ as in, do what you can when possible to know exactly what you're plugging in ......... which is good practice and training for other things we may do later ---- regardless of electronics or something else. Mistakes can and do occur. But everybody doing their best to cut down on mistakes can be beneficial.
But sure ...... things like that can happen. This is where having backups (a few arduino UNO) for spares ...... is also good heheh. With 1 or 2 spares ...... if these ones continually behave properly, while the one in question stays 'on the blink' (with the LED that is), then maybe the one in question could really be damaged ----- if it consistently shows the same symptoms for the same connection arrangement.
Uh, I don't know exactly but I know I've done something like this, and no damage took place on my board...
All in all, I'm not mad that the Uno may be fried. I've had it for over a year now and it's served its purpose as far as I'm concerned. I was planning on buying a new one eventually for new projects, so it's not a big issue. Dumb mistake on my part, but hey, mistakes happen to even the best of us.
I will update this post periodically to relay the condition of the Uno: if it gets worse or maintains function. I may modify the Uno and take a 100k ohm resistor and solder it to ground to render the D13 pin unusable.
Hi,
Did you connect the wire WHILE YOU HAD POWER CONNECTED TO THE UNO?
Tom.... :o :o :o
dominicj:
I was planning on buying a new one eventually for new projects,
Please don't!
Get a Nano - or multiple Nanos - clones are cheap and good. Much more practical than the UNO.
dominicj:
I may modify the UNO and take a 100k ohm resistor and solder it to ground to render the D13 pin unusable.
Why would you want to do that?
I doubt the described mistake could damage Arduino (or the sensor). I cannot see any way how it could cause symptoms described.
Update, the Arduino is still functional. It's being used for a fairly complex medical research project and is performing as expected. Apparently, pin 13 was fried, but otherwise, the board acts as normal.
Paul__B:
Please don't!Get a Nano - or multiple Nanos
Nahhh... Nano Every is better. Get a pack of 3 from Arduino.