Sometimes I needed to cancel all workflow on a specific list.
The following powershell script will help you to do this.
#Site URL
$web = Get-SPWeb "http://urlforsite.com";
$web.AllowUnsafeUpdates = $true;
#List Name
$list = $web.Lists["ListName"];
# Iterate through all Items in List and all Workflows on Items.
foreach ($item in $list.Items)
{
foreach ($wf in $item.Workflows)
{
#Cancel Workflows
[Microsoft.SharePoint.Workflow.SPWorkflowManager]::CancelWorkflow($wf);
}
}
$web.Dispose();
How to cancel all workflows on a list

This really should be a ui option on the list. Cancel all errored workflows.
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