Question: Anyone have the problem with Undo operation (Cmd + Z) in Xcode?
When I am edit my Objective-C code with Undo operation, some symbols are not correctly edited.
For instance:
was {   

with Undo {{   

XCode 4.2.1

Answer: 
I have the same issue with Xcode 4.3.2, specifically the screen starts going garbled and if you are showing line numbers in your editor those will get messed up to then click somewhere else and *crash*.

Few quick helpful tips I have found:

1. Option-Command-S *before* you Command-Z (Save All) [in case it crashes you don't lose work]
2. If you do undo and you see it's messing up, open up another text file, then click back on the messed up file and it's now all better.

I sure hope Apple fixes this fast!
Go slowly when using Undo/Redo. I've noticed that it crashes more when I'm hitting Command-Z many times quickly. 

Try turning off the line numbers in Xcode preferences. That seems to have improved things on my Mac. 

Preferences -> "Text Editing" tab -> "Editing" subtab
 uncheck the "Line Numbers" button

From the look of the crash logs, it may be some kind of combination of undoing/redoing (editing) causing the view to scroll or the layout to change dramatically.

From my crash logs:

UNCAUGHT EXCEPTION (NSInternalInconsistencyException): -[DVTLayoutManager \_fillLayoutHoleForCharacterRange:desiredNumberOfLines:isSoft:] *\** attempted layout while textStorage is editing. It is not valid to cause the layoutManager to do layout while the textStorage is editing (ie the textStorage has been sent a beginEditing message without a matching endEditing.)