I recently tried to buy two songs from iTunes on my iPhone 3G. Both songs errored out and listed in the "Downloads" tab with the error "Download error. Tap to retry." The problem is that no matter how many times I tried, it failed and returned to this error. After searching Apple's forum, the most consistant fix was to plug your iPhone into your computer, launch iTunes, and iTunes will find and finish the download. My problem is that I was traveling and didn't have access to my home computer to accomplish this. After further research, I found this thread:
Download error. Tap to retry.
came up with an answer which worked and did not require the mobile player to be hooked up to a computer with iTunes to correct the issue.
1. Turn off Wifi
For some reason, if the iPhone searches for or finds a Wifi connection during a download, it can cause this process to not work.
2. Return to iTunes / More / Downloads
When you get here, you should once again see the song(s) which didn't download, still with the annoying "Download error. Tap to retry." message.
3. Tap to start the download
It is important to be quick about proceeding to step 4.
4. IMMEDIATELY tap again to pause (this may take multiple tries)
I had to try to do this several times to get it paused fast enough. One suggestion (which I didn't try) was to slow the iPhone down by playing a song while trying to do this.
5. This will remove the song from the list
I can't find a reason that this causes the song to disappear, but it does.
6. Exit iTunes and go back into the application
While this is rather self explanitory, you have to exit the iTunes application on your iPhone before you can restart the download.
7. Your song download should now say 'Tap to resume download' instead
This process clears out the partial download, so you will have to download the song in its entirety.
8. Tap the song to download
Again, this will start the download of the complete song, rather than trying to pick up the download where it had left off last time.
9. Once complete, exit and turn Wifi back on
This is, of course, optional. You can leave Wifi off if you so desire.
This has worked every time for me and gives an alternative to hooking up to a computer to correct the issue.