What gets me about manifold bolts is when you want them out they are next to impossible a lot of the time and the stud breaks in the head
Never had one actually come off, but have seen them loosen in the past. Usually if installing the headers after a few runs they loosen off, possibly a result of the gasket settling in? Not really sure but its happened
Agree with Dukedesmo, although the nature of vibrations on my Ducati 1098 seems to make that bike more prone to shedding fasteners than the Guzzi. Nearly lost the rear heatshield when on one run 2 out of 3 fasteners were ejected and the remaining was very loose. Became aware of it when the shield had enough movement to start resonating and the noise drew my attention to it, remember being utterly amazed that I could loose 2 out of 3 bolts in 200 miles
That said I've seen vibrations on Guzzis break, steering damper brackets (Daytona), Fuel pump brackets (Daytona), coil brackets (Sporti), rear shock remote resevoir brackets (Daytona)