Not Wearing It Correctly
You can wear your Apple Watch pretty much any way you want — and a comfortable fit largely comes down to personal preference. That being said, there are certainly some downsides to wearing an Apple Watch incorrectly: it can result in inaccurate movement and heart rate data.
The perfect fit for good heart rate data should be snug but not overly tight. A good rule is to tighten it as much as you can without it being uncomfortable. You may also want to wear it fairly high on your arm, as wearing it too low could cause problems with comfort and heart rate accuracy.