Projects suddenly missing in C# solution

Noticed a strange thing today.

I'm working on a solution that has 开发者_高级运维about 20 projects in it - mostly DLL, three WinFowms applications. It is several years old, was started under .Net Framework 1.1 and then successfully migrated under .Net 2.0 and is now opened in VS2008.

Today I noticed that some of its projects just disappeared from the Solution Tree. I didn't delete them, folders were at place and adding them back even didn't change their source control status. Though I had many unpleasant minutes trying to understand why one of the apps is not working correctly... And it had happened twice today!

All this is very annoying and seems to be a mystery to me. Any suggestions why this could happen and what can be done to prevent such situations?