I don't think I'd be worried about it, but if she still comes up lame the second time around, the way I see it, you've got 2 options.
1) You can stop considering this mare for purchase and continue your search
2) Spend more money trying to find the source of the lameness; x-rays, maybe even MRIs, etc. Then, if they are able to find the source (which they may not be able to) and decide if it's treatable (which it may not be), you'd have to decide if it's something worth dealing with or not.
You never know, it could be something as relatively minor as an abscess brewing or a pulled muscle from playing in her paddock. It could be something as serious as navicular or early arthritic changes.