Symptoms

When pulling from, pushing or cloning to Bitbucket Server, it responds with the error code 503.

$ git clone url directory
Cloning into 'directory'...
error: The requested URL returned error: 503 while accessing url
fatal: HTTP request failed

Cause

This may be caused by one of two things:

1. If you have a proxy set up in front of Bitbucket Server, it may be returning the 503 due to Bitbucket Server being down.
2. If you don't have a proxy and you are using Bitbucket Server 1.0.0, it is likely that you are hitting internal Bitbucket Server resource limits.
   In Bitbucket Server 1.0.1 and above, you will get an explicit error message in the git command output if you hit the resource limits. 

Resolution

When you visit Bitbucket Server in your browser, it will add a banner if it is under extreme load (confirming cause #2 above). You will have to wait for the load to decrease - try again occasionally.

If you are hitting resource limits, you can configure those limits in Bitbucket Server, as documented in Scaling Bitbucket Data Center.