It usually means that a Url for a media asset is missing the Hash query string parameter when being called.
This was introduced in v7.5. It attempts to fix an image resize vunerability where you could potentially fill a disk with different image sizes.
If you use a
FieldRenderer you will get a valid url with the Hash query string appended.
If you just generate the url, you will need to add the hash. Fortunately Sitecore has some utilities to do that:
Sitecore.Resources.Media.HashingUtils.ProtectAssetUrl : will return the full Url with the hash added.
Language = Context.Language,
Width = 100,
Height = 75