Manual Mode of the camera or Manual Flash mode? Different things. What do you mean with "i use 1/8"? 1/8s shutter speed or 1/8 flash power?
If you use flash power 1/8 and (based on all other settings like Aperture and ISO) the resulting flash speed is above 1/200s then the camera will set the speed to 1/200 and the shutter speed display in viewfinder will blink (Canon). Reason is that 1/200s is the maximum X-sync speed for flashes in normal mode. You can take images but the result might not be what you intended.
That's the upper limit. Whether this is correct for your picture and intention doesn't matter for the camera. It's a technical limit in this mode.
You might want to read more: http://www.photonotes.org/articles/eos-flash/