'File contains unknown nodes or data'

Some people happens to run into a problem when trying to save a Maya Binary (.MB) file to Maya ASCII (.MA) file. An error message would pop up mentioning, 'File contains unknown nodes or data. To preserve this information, the current file type cannot be changed.'

In this case, you can write this MEL script to look for the 'unknown' node(s):

ls -type unknown;

A list of unknown node(s) will be generated.
You can delete them by typing:

delete `ls -type unknown`;

By deleting the unknown nodes, the problem should be solved. =]

 UPDATED 12.01.2010 :
Thank you to Ariel for adding extra info.
If it says "Cannot delete locked node", make sure you get the name of that node and select it, and then type:

lockNode -l 0;

Then type again:

ls -type unknown;

delete `ls -type unknown`;

That should do it.