I've had this in the past where n-gons were involved (i.e. polygons with > 4 sides). Is this your own modelling, a freebie, or a commercial item? In the former two cases, I could have a look at it.

It's actually a conforming dress from Poserworld that I'm trying to convert to dynamic but thanks anyway.

I may have it, if it's not too recent.

I don't think it's too recent, it's  the Mia dress.  I did nothing to it but import the .obj to use as dynamic and constrained the straps.

if you export the defected dress and import it to a 3d app, or back to poser you can get a better idea of what is causing it (its porbably a unwelded vert or ploy)

Curious - I have Mia tops and skirts for various figures from PW, not dresses, and they're dynamic already. Plainly not the same thing... I don't recognise that dress, but I do have a load of PW stuff so who knows?

That's strange, I don't have any other Mia clothing just the dress :)  It's on this page

http://poserworld.com/Downloads/Victoria.4/10628/Mia.for.Victoria.4.aspx

and the zip's name is Mia-V4.

cfpage, I'll try importing it with the weld option in Poser, maybe that will help.  

Two misbehaving vertices, as I can see. Put them back in rank :)

But why the trouble with the obj? Just give her the conforming dress, use conform to. Then clothify a part of the dress, eg the skirt might do. Maybe it's a material group already, easy to pick from the list in the group editor.

http://poserworld.com/Downloads/Other.Figures/9398/Mia.set.aspx

I don't have the V4 Mia Dress, sorry.

Anyway... Do you have UVMapper Pro? If so, open the geometry file in there, and do Select > Facets > N-sided, then choose 'More than' and 4 sides. That will tell you if there are any >4-sided polys, which is what I would have looked for had I owned the clothing in question... (deep breath)

For dynamic simulation, triangular mesh is best; UVMP can do that for you too, along with any welding that may be needed.

I'm a bit surprised that the dress isn't set up for dynamic (or at least hybrid) use, since Steve generally makes dresses and skirts that way nowadays. Just seen aRtBee's post, I see we are of the same mind. :)

aRtBee, I was originally going to use the conforming version and just clothify the hip part as I usually do but I've used many corrective morphs for the upper arms and collars that the straps just woudn't behave so I figured "why not make the whole thing dynamic?".   At some point I did try your suggestion but the skirt part separated from the rest of the dress. 

EnglishBob, thank you, unfortunately I only have the free version but will try saving the .obj through that as I recall that fixes some things.

In any case, I just ended up using the dress as a conforming item and it didn't turn out that bad :)