Apparently none has mentionned that one yet.
Often when right-clicking on a stack to fortify all of them, units for which I pressed space before (mistake) are written as fortified but wake up automatically next turn. SO have to do Fortify again.
Or is it to avoid the exploit of bringing a unit, activating another one then fortifying them both.

Probably to prevent the exploit. I have noticed this one. It does give the appearance of fortifying the unit if you choose the fortify option, but I don't know if you get the fortify bonus in those instances. If you right clcik on the stack after fortifying, it will show Wake All as an option even though next turn the unit would be active as you mentioned.

Was that right-click/fortify-all thingie for a mixed stack with units having 0mp and 1(+)mp a confirmed exploit in vanilla/PTW at all? I mean, did those 0mp units really get the fortify bonus?

According to Soren, they never actually got the fortify combat bonus, just the animation and the indication in right-click menus that they were fortified.

OK then, thanks for the infos.

Anyhow, I might test it for C3C some day, since units show the 'wake' command instead of 'activate' (the latter would make more sense IMO).