When booking in orders through Shiptheory, some of the order references may contain a hash key (for example; #123456), this in itself is not a problem, we see many customers use this and as such, there shouldn't be any overall issues there.
However, there is one issue that can crop up when using the Shiptheory API in conjunction with references of this type; that's when using the GET Shipment request, also known as "view shipment". Please see here for some more information: View Shipment
The issue here being that the hash causes an issue in the GET URL. As such, it needs to be made "URL Safe", this can be done by encoding the "#" key in your request to a "%" key, this should yield the desired results. See below for an example of this;
Of course, if you have any issues from there, please feel free to Contact Support.