When trying to fetch a PNG from textures.minecraft.net with Cloudflare Workers, you sometimes get a 520 HTTP status code. This is the status code that Cloudflare gives to subrequests that return an empty, unknown or unexpected response. 

You can test this in the Cloudflare Workers playground. I made a snippet available here on the playground where you can click Send at the top right to trigger the worker. You will see if the request for a skin texture was successful after that.

It might work fine, the subrequest to textures.minecraft.net returns HTTP status 200 and the PNG. However, if you try a few more times you will see that the subrequest will return HTTP status 520.