Some servers disconnect a git clone when too many concurrent connections are launched at once, which results in the job failing with a GitException. This is unfortunate, since it could instead retry a few minutes later and end up with a successful run.

Example output of the exception:
ERROR: Error fetching remote repo '<snip>'
hudson.plugins.git.GitException: Failed to fetch from <server>
at hudson.plugins.git.GitSCM.fetchFrom(GitSCM.java:812)
at hudson.plugins.git.GitSCM.retrieveChanges(GitSCM.java:1079)
at hudson.plugins.git.GitSCM.checkout(GitSCM.java:1110)
at org.jenkinsci.plugins.workflow.steps.scm.SCMStep.checkout(SCMStep.java:113)
at org.jenkinsci.plugins.workflow.cps.CpsScmFlowDefinition.create(CpsScmFlowDefinition.java:130)
at org.jenkinsci.plugins.workflow.cps.CpsScmFlowDefinition.create(CpsScmFlowDefinition.java:59)
at org.jenkinsci.plugins.workflow.job.WorkflowRun.run(WorkflowRun.java:262)
at hudson.model.ResourceController.execute(ResourceController.java:97)
at hudson.model.Executor.run(Executor.java:405)
Caused by: hudson.plugins.git.GitException: Command "git fetch --tags --progress <server> refs/changes/<change>" returned status code 128:
stdout:
stderr: key_load_public: invalid format
Received disconnect from <ip address> port <port>: Too many concurrent connections
Connection to <server> closed by remote host.
fatal: Could not read from remote repository.