Failed to launch simulator: Operation failed with underlying error 4294956467.Trying to run an application under development fails, because simulator crashes. The error note is totally useless, unfortunately. Callstack seems to refer to an Interface Builder file containing an IBOutlet, which was removed some time ago. Doing "grep -r keyword *" in project folder can't find a thing, just some references under .svn subfolders. Shouldn't be a problem, version control is supposed to remember old forgotten things.
Problem seems to be related only to simulator, not to current source code. Doing "Clean" from Xcode menus doesn't seem to be enough, still keep getting same crash. Got to get more aggressive, so let's try deleting some folders. In this computer(*) simulator files are located at:
/Users/ext-mietjo/Library/Application Support/iPhone Simulator/4.0.2/Applications/That folder contains several simulator subfolders with weird long numeric titles. Each subfolder seems to contain an application, so check all and delete your crashing application. Since controlling which simulator opens by default is not very clear, you might check all installed simulators and delete your app subfolder under each.
If you know what you're doing (I sure don't), might as well delete all app subfolders (I did).
Just a note that I have both Xcode 3.2.5 and Xcode 4.0.2 installed. Even thought they are installed in different folders (/Developer and /Xcode4), I'm not quite sure how simulator files under my home folder are related to either one. Actually I believe that since I use both Xcodes to develop and run the same application each day, that might be the reason for these problems. Will remove older Xcode once the new one stops crashing, adds some missing functionality and increases usability a lot. Hopefully during this year.
Fingers crossed, let's hope I don't need to update this tip! Otherwise need to figure out something completely different...
* Yes, I work as an external subcontractor, developing iPhone software. You want to hire me, contact my boss.