Unable to download podcast after iTunes previously crashed while downloading it
Go to folder where you store podcast episodes you've subcribed to. (By default it's something like "itunes" folder in "mymusic" folder). 

Search for folder named "Downloads". That folder is used for storing temporary files while downloading particulary episode, and it should be empty. 

If ITunes crashes, some parts of the episode remain in this folder and that's causing the problem you encountered. If happens that there are some folders or files with .tmp extension, delete them and then try again to download particular episode.