I thought that I would commute by bicycle today, but I got only 0.5 km away from home when my bicycle broke. The acorn nut in the rear axle came loose, and as the thread had got stripped in the nut, it wouldn't stay on. So, I had to walk the bicycle back home, and commuted by car instead.
This evening I tried switching the left side nut to the right, and it went on perfectly. But the stripped nut was so loose that there was nothing else to do that try to find a shop selling replacement nuts for the Shimano axle.
Apparently Shimano Nexus 8 axle nuts are 3/8 inch 26 tpi, which is an old thread standard (BSC), no longer as popular as it used to be.
I found that I'm not alone with the problem, as the flat spots on the Nexus axle are rough on the nuts. The axle is of much harder material than the nut, and thus stripping happens if one is not careful when installing the nut. It may be also that when one stresses the rear tire heavily (going up and down steep hills etc.) the nut thread gets stripped.
There is a bicycle shop in Helsinki selling Shimano spare parts, and this nut costs there 4.30 euro, and they have it in store. If it fits, the repair is cheap, even though 4.30 euro is a steep price for just one nut.
(Posting title is from the poem Lullaby by John Fuller.)