SW 2010 latest SP, Windows 7 64 bit, blah blah
I design die sets - match metal, sharp-edged..
I usually design from the assembly - start with blank assembly, create some base 2d sketches to lay some things out, and build new parts in-place (I sometimes keep the inplace mates, often remove and create conventional mates instead, depending on the part.)
Early in the design process, I have no issues - I create my mates, relationships and so-forth as I go, and things remain stable throughout any changes I may make, opening, closing the assembly and whatnot.
However, at some point - and I hasten to add this is not a 2010 issue - it's been happening since 2008 release - I reach some critical mass - usually when I begin adding mounting features - bolt holes, dowel holes, etc..- where Solidworks says, 'That's it, screw you, I'm going to start randomly forgetting external links in your 2d sketch relationships.
Now maybe I'm going about things wrong - I'm self-taught, so maybe I never got told the 10 commandments of bug-free Solidworks useage. If I have two parts that align together, They'll either be in-place mated, or what-have-you, but they will be fixed in place somehow (I dislike "fixing" the part, so generally fixed means 3 or more mates, or an in-place.) and I will add a 2-d hole wizard feature to one of the parts, for, say, c'bored holes for screws. I will next edit the mating part, select the appropriate face, another 2-d hole feature, and quick-snap to the centerpoints of the holes in the other part. This seems logical to me - I have only one feature to change if I need to move holes, then a rebuild, and all my holes update, any mated screws move, and I'm good.
But at some point after I've done those features, this could be a day, a week after, I'll open the assembly (knowing that it was previously saved in a "good, re-built" condition) - and I'll have errors. One of those coincident relations will be "no longer valid" or missing, and I'll have to go into the offending feature, delete and re-create the same relationship. Sometimes there's just one, sometimes 5, but ALL generally at random, unrelated errors. I can make a change to some other area of my assembly, completely unrelated, and it'll screw something else up.
I know you're saying, "well maybe you just don't know what you're doing, too many relations, etc.. making changes that will affect things and not realizing.." - but understand, it's always just one random relation in a feature sketch, NOT the entire set of relations - if I have 8 holes in a feature, 1 hole will be hosed up, the others are fine.
Any how, enough ranting - maybe this is a common issue with SW, I've learned to just deal with it. But it is scary when I go back into an assembly that I've already detailed the parts to, and have to remember exactly what I was wanted to relate to what 2 weeks previously.