If you see "Yon 4chan viewer" could not be downloaded due to an error (500), plese try below:
Make sure you've cleared the cache and data of both the Market app and the Download manager. Here's how:
- Visit Menu > Settings > Applications > Manage applications
- Tab over to "All"
- Select the Market app, and then tap "Force stop", "Clear data" and "Clear cache;" then, select Download manager and tap "Force stop", "Clear data" and "Clear cache"