Ok, here goes.
some mags WILL fit an extra round over what they are specced for. Sometimes this is because they cahnged something since the spec was set out. For example CZ75 15 and 16 round mags, there was a follower change and for a while you could buy "15" round mags, and they all seemed to hold 16.
This is not the case with the M&P. The reason you can cram an extra round in theres is likely because you are doing one of several things.
First, and most probably, is that you are muscling the last round in, and slightly distorting the feed lips. Over time, this will likely cause malfunctions later because the feed lips have become out of spec. Common results form this in a pistol would be having a live round flip up 2with the case rim still retained by the feed lips when trying to feed. You will note in your picture, the round does not appear to be flush with the rear of the mag. Thus the rear will have proper tension, and the front won't. This usually results in a death jam, where you have an extracting round and a live round vying to occupy the same space. To clear it you ahve to brute force the mag and the gun apart.
Second, in a magazine design liek the M&P where it is basically a single stack at the top and a double farther down, you can cause the single stak width coils to kink and bend over in the double wide section of the mag tube. This leads to spring fatigue at least. It can also cause the follower to tilt over and jam in the mag tube. Given the M&P design, I don't think this failure mode is terribly likely as the desing mitigates some of these issues (i.e. length of the single stack widht coiled section of the spring, anti-tilt legs on the follower, etc.
Then of course you have the problems it may cause from simply not operating the thing right. If you notice, the cartridge doesn't seem to fit flush with the rear. Ths can cause you to be unable to seat the mag in the gun when the slide is forward. you reload with this, and you may get a failure to xrtract because the rear of the extracting shell catches on the edge of the improperly positioned bullet. Etc. Then of course there is the ammount of pressure on the spring. If it is completely compressed, there is no give other than the mag lips. Which only give upwards, not downwards. You jam it in the gun and make it latch, you may actually cause a jam from too much friction on the slide. Have you tried shooting with your mags loaded to 18? Take apart your gun and check the bottom of the slide for a nice brass stripe. If you see one, think about where that is coming from and why. Then of course, there's the worst case scenario. That you are shooting something that is a hot round, and you ARE causing interference, but rather than a jam, you start getting bullet setback on your very hot +P+ round. 9mm is a nice strong case, but you could still get a nice kaboom, or a blown primer and some hot gasses back in your face out the rear end of the gun.
All of the above are conditions taken from real world guns except the kaboom/hot gasses. The person that happened to was forunately shooting mosue fart loads, so all they got was some extraction issues from the increased pressure and some really flat primers.
The guns that contributed to the above list of crap that can happen due to mag problems are STi .40 mags with .38 super springs in them, and other fat 38 super mags without proper tolerances. STI ls-9, walther p99, cz 75b, various 1911s in .45 acp, and a few others I can't recall off the top of my head.
However, you are free to do whatever the hell you want. But I'll piint out that if round 18 is not flush againt the back of the mag, it DOES NOT FIT. Despite what you would like to believe.
ETA: Oh, and the reason why S&W says they it won't hurt is that they don't consider it warranty voiding behavior, and don't know you will actually harm anything in particular. So if you break it, just send it in. That's WAY different than being told.. yeah it works just fine. If it worked just fine, it'd be in the manual and listed as the specced capacity by the engineers who designed it. What you know is that it isn't on the list of things labeled "DO NOT DO" for the CS FAQ.