The chain adjustment on many bikes consists of a carrier with a threaded stud extending to the rear with an adjustment nut and jamb nut to hold it at the desired position.
On my Ninja 650R, the nuts are two different sizes across flats, the larger being for adjustment, the smaller for locking the first. Well, I have adjusted the chain a few times, and I changed it for new once, so each time, I have had to adjust these nuts.
The first couple of times, I used a standard box-end wrench from my toolbox. Unfortunately, the thickness of the larger wrench is greater than the thickness of the large nut. This caused the smaller wrench on the small nut to slip off, rounding its corners.
In hindsight -- always 20/20 -- I should have used the wrenches in the bike's toolkit. Duh.
Not wanting to show off my ineptness to every passerby, I replaced the damaged nuts the last time I had the rear wheel off for a tire change and chain adjustment.
To avoid a repeat performance, I bought a couple of wrenches specifically for making this adjustment, one 12mm, and the other 14mm. The larger of the new wrenches was also too thick, and would have caused the same problem. I solved this issue by grinding down the thickness of the larger wrench to match the thickness of that nut: The left side of the right hand wrench in the picture below.
Here is a shot of the wrenches in place for adjustment.
I found a small plastic box to store the wrenches in, and I put a card in there with them showing the socket sizes of the axle and its castle nut, the range of chain play that is acceptable, and the torque to which the axle nut should be tightened.
That saves me from digging in the service manual every time I have to make an adjustment. You can see I have fingered it several times with my greasy digits.
This pair of wrenches that fit properly works much better. The small wrench doesn't slip off now, and my new hardware remains like new.