Xcode compile error

-symbol(s) not found for architecture i386

Have had this error a number of times at compile time.  There are no apparent errors or warning prior to compile, but it fails nonetheless.
You need to read the bottom portion carefully for clues.  This time it said CoreDataTableViewController was not found.  I deleted these files and re-imported them, and it worked!
Go figure, apparently the original import routine (drag files to file organizer) didn't work.

duplicate symbol _OBJC_IVAR_$ - Xcode linking error

I received this error today after inputing Apple's "Your third iOS Program", SimpleTextEditor.xcodeproj.
After struggling thru the error message, and replacing a lot of the code with cut/pastes from Apple, I went to their forum, and found the problem.

I mistyped this statement - #import "STESimpleTextDocument.m" as an import for the DetailViewController header file. It should be .h

 Forums rock!