During version creation, JIRA considers versions to be case insensitive. That is JIRA sees both TesT and test to be the same. If TesT already exists as a project version, then the admin cannot create a test version.

During a REST call, however, when you make a reference to a version, you must match it's casing exactly.

For example, making a REST call to TesT version will work fine, but making a call to test version will result in:

{"errorMessages":[],"errors":{"versions":"Version name 'test' is not valid"}}