An overlapped extent allocation is a Volume Structure problem. Volume structure errors are serious. They indicate corruption in the disk directories. First, be sure anything important is backed up right away if possible to be safe. You can try using the Data Recovery routines in TechTool Pro 4 if the volume does not mount. Then go ahead and try a Volume Structure repair.

Here is some information about the overlapped extent allocation problem:

Each instance of the overlapped extent allocation error means that one sector on the disk is recorded as belonging to two files. Only one of the files should have the data in that sector in it. If the Volume Structures test in TechTool Pro cannot fix this problem, you may be able to find out the identity of one or both of the files by running fsck in single-user mode at startup. In some cases, fsck reports two numbers, and in some it reports one. It is not absolutely certain that cases in which two numbers are reported are cases in which both files in the same overlap are being identified, but it seems that that is what is happening. (The other possible interpretation is that one file in each of two overlaps is being reported by fsck.) If one or both of the files can be replaced from a backup, you have a successful resolution.

The overlapped extent allocation problem affects all computer platforms and all volume formats. Something prevented the disk directory from being properly updated.

Apple Knowledge Base article 106214 provides instructions for running fsck in single user mode:
Using Disk Utility and fsck for file system maintenance in Mac OS X.

Some cases of this problem are not able to be fixed by anything, because there is no way to determine which file should have the disputed sector in it.

Here are some instructions for identifying files by inode numbers:

Sometimes fsck reports one inode number when it diagnoses an overlapped extent allocation, and sometimes it reports two. Be sure to write down the inode numbers that fsck reports.

It would be much better if fsck reported both inode numbers. Eliminating one of the files is helpful, but only if it is the damaged one.

Here are the steps to find the file, using its inode number:

When you run fsck, it should report both the inode number of the file, and its name. If the inode number is nnnnn, follow these steps to get the pathname to the file:

Open Terminal, located in /Applications/Utilities. Type:

sudo -s

Press Return. At the prompt, enter your administrative password, the one you chose when you installed Mac OS X.

Type:

find / -inum nnnnn -print

If the reported inode number has a letter at the end (often a d), omit the letter.

Inode numbers are unique only on a given volume, so a search for a particular inode number may return results from volumes other than the startup volume, which is the only one that fsck examines when you run it in single user mode. You can ignore the files on other volumes that happen to have the same inode number.