CloudFormation "CREATE_FAILED ... already exists" in CI

CloudFormation tried to create a resource whose physical name is already taken (an existing S3 bucket, IAM role, or log group), so it marks the resource CREATE_FAILED and rolls the stack back.

What this error means

A stack event shows status CREATE_FAILED with a reason like "<name> already exists" or "Resource of type ... with identifier ... already exists", and the stack ends in ROLLBACK_COMPLETE.

CloudFormation

CREATE_FAILED   AWS::S3::Bucket   AssetsBucket
Resource handler returned message: "my-app-assets already exists"
(Service: S3, Status Code: 409, Request ID: ...)

Common causes

A hardcoded name collides with an existing resource

The template sets an explicit BucketName or RoleName that already exists in the account or globally (S3 names are global), so creation fails.

A previous failed deploy left the resource behind

A resource was created outside CloudFormation, or a prior stack was deleted without removing it, so the new create hits a name clash.

How to fix it

Let CloudFormation generate the name

Remove the explicit name property so CloudFormation assigns a unique physical ID.
If a stable name is required, make it unique per stack with !Sub and the stack name.
Redeploy so the create no longer collides.

template.yml

Resources:
  AssetsBucket:
    Type: AWS::S3::Bucket
    Properties:
      BucketName: !Sub '${AWS::StackName}-assets'

Import or delete the orphaned resource

If the resource already exists and should be managed by this stack, import it; otherwise delete the orphan, then redeploy.

Terminal

aws cloudformation create-change-set \
  --stack-name my-app --change-set-type IMPORT \
  --resources-to-import file://import.json --template-body file://template.yml

How to prevent it

Avoid hardcoded physical names; let CloudFormation generate them.
Scope explicit names per stack with the stack name prefix.
Clean up resources created outside the stack before deploying.