Q: What is Error 36 and how do I get rid of it?!

As far as I know there isn't an error 36.

There is a -36 (it does make a difference)

Apple Error codes - http://support.apple.com/kb/HT1618 - -36 ioErr I/O error (bummers)

Did this happen in the middle of a download, or did you happen to notice it while doing a download but after doing something else extensive with the computer? 

Do you keep your library on an external drive?

The classic reasons for a -36 would be drive failure or corrupt formatting, or a bad cable/connection. While those are less likely with newer computers it may still be possible. If it had happened before you started using iTunes and you had just done something to the computer I suppose it could be bad permissions on a folder blocking access though usually the error says something such as "cannot access folder" blah-blah.

All my references for -36 are fairly old but I don't think -36 has changed in years (otherwise programmers would have to remember exactly when a code stopped meaning A and meant B).  Let's hope the reference is to the other end of things, namely the connection to the outside and there was a burp in your Internet connection. I would try downloading it again (you may need to delete partial downloads completely first).

How old is this computer? Drive failures do happen to people, though I have been fortunate with my 8 year old drives (back when they made them to last).

Oh, there's also someplace in Disk Utility where you can pull up the SMART status of a drive. I don't know how truly reliable it is at indicating drive state.  I think it gets triggered if the computer has to map out an excessive number of bad sectors.

Basically -36 is saying,  "I am having problems communicating with a device."  Sometimes it means your hard drive is about to fail but then it should be appearing under many situations. One way to see if a hard drive is about to fail is to check the SMART status of the drive. there is an application called Disk Utility in your Utilities folder which does that.  If you don't want to do it then find somebody who can.  Or wait until your drive fails and you lose everything and then don't have to worry about the error when you start off with a brand new completely empty drive.  It isn't necessarily the drive but you have to start somewhere.

2011 = 4 year old drive. A bit early for it to fail but it is middle-age by now so it may be.  Or, as I said, it could be something else.  It is difficult, even with SMART testing, to tell when a drive will fail, hence the need for a constant backup.  Or it could be a loose cable.  That's the problem with the general nature of -36, just 'something isn't right in trying to achieve the task requested'.