A few weeks ago I purchased 4x NCR 18650B button top batteries. I decided to charge them separately and also decided to charge two of them in series. Before the charge one battery was at a charge of 4.1v and the other was at 3.6v. I checked them after an hour or so and discovered the battery which had been at 4.1 was no at a mind numbing 4.67v and running hot to the touch, the second battery was at 4.2 volts. Could that have damaged my battery?
4.67V? yes, you did damage it 
for your own safety do not use it, dispose of it safely
What charger?
It’s gone. I hope you don’t try to salvage this one.
Expensive lesson - don’t serial charge without balancing. Or easier - just use parallel charging - this type of overcharge doesn’t happen that way.
I am also curious how you tried to charge them, a real charger does not allow series charging (never heard of anyone trying to charge batteries in series at all, for that matter). What did you use?
The charger is an imax acb6 charger.
I think I will stick to parallel
You’re lucky it was a genuine Panny cell. Had it been an unprotected Trustfire/Ultrafire/XXXFire, you may have lost your house!
Even if you are going to charge them in parallel you have to make sure that the voltages are very similar to start with. 3.6 - 4.1 is too large a disparity.
IMV charge batteries in sets (as they are installed in the devices one is using), and keep an eye on the individual cells voltages
+1
As others said, luckily its a Genuine Panasonic cell you abusing 
Why not get a Nitecore i4 or Xtar VP1 ? It’s really much much easier to charge and use that hobby charger for capacity check (discharge) 
I made one other mistake in trying to get the voltage down. I tied all four batteries together in parallel hoping they would balance each others voltage without any input. So far that worked but now I do not know which battery was to high.
Now you are in the proverbial Do a internal resistance test on each cell. Get a brick and metal box, or stick the brick outside on a window ledge. do a discharge test on each cell sitting on top of the brick (the battery! though with your luck!) :Sp . Should give you a clue.
Problem is I have been doing (or was doing till the shipping fiasco) abuse testing of cells and with pans they can take an enormous amount of abuse without really showing it in the short term so that cell might be difficult to detect, BUT that does not mean that it is safe especially in the long term and without careful monitoring.
As you will now have gathered best to mark each battery as a set and individually.
Abuse testing. Very interested.
It looks like no damage occurred. All the batteries act the same. Even the resistances are really close 10.99-11.02 M ohms. My last test will be a discharge test to see how much capacity each battery has.
I have a genuine panny cell that I ripped out of a tool pack, low capacity however high discharge and a very safe cell in general akin to IMR’s (although, different).
I accented shorted that cell one day on a p60 build (Reflector short, bad insulator) and I realized it very quickly and ripped it right out it still left the light fairly warm it it was fairly warm itself, the spring was near scalding 0.o. I labeled it bad and left it in a fireproof box for a few days (Yes it did not short out in the box, I had some plastic holding it in place) and checked it later - Normal readings.
So I tried it in a light, and it worked, no problems, no smoke, nothing. Threw it in my jerry-rigged smart charger and it said no go, internal resistance fault; checked it again and it was fine. Threw it in a i4 and it charged.
Just saying some of these cells can be damaged and still useable however that may not be safe or practical. Not every piece of equipment is going to pick up on this, my accucel did however my i4 did not.
Still works find though, I’ll give you the heads up if it ever blows 
(Obviously not using it in series, have two of the same cell types in a series light with no issues but they’re balanced and matched pairs).
Just a heads up, the cell may be dead and you don’t know it while you continue to use it. Just watch out and you should be fine (Although no more series, that’s just a bad idea waiting to happen; if you need to do multiple cells make a balance tap and use that to charge with).
No such luck. Some damage occurred. It could be trivial, or it might simply be hidden. You made a fairly significant mistake in serial charging cells of extreme voltage difference. Do you know enough to to tell if this cell is actually safe now?
As I said, from my tests it may be difficult to establish which cell that was overcharged. But please do not be under the impression that NO damage has been done and the cell is SAFE.
I am one of the least likely people on this forum to overreact about cell safety but in normal circumstances that cell is toast, it really is not worth worrying about the few quid for a replacement.
If you can’t establish which one of the set of cells, was the one overcharged, I would recommend binning the lot. Compare it to a round of drinks at the pub pricewise.
I know the results of my tests so far, and I am deliberately doing (in using cells outside their safety parameters). what I am suggesting you don’t do, but I am monitoring the situation very carefully and know full well the dangers involved. Even if the cells tests ok in the short-term, I reiterate it does not mean it is safe, either over the short or long term.
If you are considering keeping the cells at the very least only use them as singles and never in multiple cell situations.
Always charge them individually and get a temp probe for your charger to make sure the temps keep within safe boundaries
The shipping situation really put a big mokker on this, but some very interesting results so far. But it is in the very early stages in reality. The results will never be definitive and obviously limited, and will take a long time but I think will be worth it in the end (at least to me )
Is saving $10-40 really worth risking an explosion/fire that could ruin a charger and whatever is around it, ruin a flashlight, or cause bodily harm?
Take it as a relatively inexpensive lesson and replace the cell or group of cells if you didn’t mark the overcharged one.