I dont know.

I've seen a stem failure in person. The person ended up with broken collar bone and a few knocked out teeth. Whether or not this was caused by putting a 26.0 bar into a stem 25.4 or putting a 26.4 bar into a 26.0 stem or some other cause, I dont know -- the bolts ripped out. But why risk it when one could easily flip either component and avoid any future trouble? It seems most injuries I've encountered over the years that were caused by mechanical trouble were rooted in hacks that were justified the same -- "oh I'm sure it'll be fine." A friend of mine broke the bracket on his road hoods and fashioned a new one out of scrap metal -- it was fine for awhile until it suddenly snapped while riding. The brake lever fell off and was dangling by the cable and got caught in his front spokes. He endo'ed over and landed on his shoulder -- dislocated it and destroyed his front wheel and fork. Of course, he didnt have insurance and ending up paying about $5k in hospital bills. He would have better off just buying a new set of Tektros for $25 instead.

I am sure that people will be lucky and get away with 26.0 into 25.4. I am also sure that some will not be lucky and end up wrecking. I think when it comes to bicycles, it's always better to reduce risk as much as practically possible and simply using the correct fiitting components together is an easy way to reduce risk without sacrifising much.