I'll save you the trouble of reading through the entire thread that nextdesign linked ....READ BELOW

The only solution I've found to this problem is you have to close maya, delete the userprefs.mel file or delete your maya prefs folder (*1) (the one in documents on windows not sure where the mac one is), and restart maya then reopen the scene and those shader nodes should be gone.

I have filed this as a bug with autodesk and as yet have seen no reply.

It appears this is one of those bugs that is not affecting enough people for anyone to really take notice or be concerned. This is very annoying to me as I do not know what causes it to start happening, or how to fix it without deleting the maya folder or the less destructive route of deleting the userprefs.mel file.

(*1) - Be careful deleting the maya prefs folder! Make a backup first. That folder contains scripts, shelves, hotkeys, plugin specific stuff for ALL versions of Maya you have on your system. So deleting it might solve one problem and create a dozen new headaches!


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OTHER BUGS I CONTINUE TO SEE IN MAYA 2012 and 2013 THAT AUTODESK WILL NOT ACKNOWLEDGE AND BLAME ON GRAPHICS DRIVERS
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1. The screen refresh bug where the display will stop updating, the channel and outliner will both go blank and you have to close and reopen the scene to fix it.

(workaround) close and reopen the scene seems to get things in sync again. But this can happen again within seconds, minutes, or hours, or maybe I won't see it for weeks. It's maddening!

2. The error reading scene file bug. Where I get a message that there was a problem reading the scene file at line 762 (always the EXACT SAME LINE NUMBER!!!!) and the file opens and nothing is displayed. I have to press the perspective display and then the model is displayed in wireframe (4-key) mode. In some instances the grid settings are messed up.

(workaround) This is another really perplexing problem because the fix is simply closing and reopening the very same scene file!!!!