After installing a Rhino 4 service pack, some users experience the phenomenon that extrusions, lofts, etc. with kinked curves (curves that have sharp corners like polylines) no longer produce the expected polysurfaces; instead the result is one single “creased” surface, folded sharply at the kinks.
These surfaces often cause problems with other operations like trimming and Booleans, as well as exhibiting meshing artifacts. Exporting these objects to downstream programs will also likely produce bad results.
There is a setting that determines whether extrusions and the like get split at kinks (“creases”) or not when created; in V4 this setting is not directly accessible to the user (it's in the Windows registry). It is supposed to be set to “yes” automatically without user intervention… Unfortunately, due to a mysterious bug, it sometimes gets set to “no” accidentally during an install/service release, with the results outlined above.
Fortunately, you're in luck - some people have found the no-split behavior to be useful in the past, so someone created a plug-in for accessing this setting from inside Rhino so you can turn it on and off. A Windows registry edit will also work to change the setting, but installing the plug-in is far easier. The CreaseSplitting plug-in can be found here.
If you install the plug-in, you will then be able to set it as you like from inside Rhino using the CreaseSplitting command added by the plug-in. Start the command and set SplitAlongCreases to Yes for the “normal” Rhino behavior, to no if you want to produce kinked surfaces for some reason…
If you have some kinked surfaces that you want to turn into normal polysurfaces, select them and run the DivideAlongCreases command (included in Rhino 4 and 5).
In Rhino V5, the CreaseSplitting command is already included in the installation, so no plug-in is necessary.