Original Post:

In snapshot 16w39c, structures don't get saved as file, when you power the structure block with redstone. On client, it's impossible to save structures with redstone. On server the structure seem to be saved in RAM (sometimes), but after restarting the server, the structure stop to exist.

Edit: On client, the structures can be saved in RAM, if you wait long enough to save (not possible with buttons). After re-entering the world, changes to the structure disappear.

Edit2: Removed.

Edit3: Saving structures using redstone will only save it to memory instead of creating a file. (Seems to be a feature, I don't like it anyway.)
But, I still can't save via stone Button / wood button.
Example:
1. Build a pattern.
2. Save the pattern as structure using the save button (UI).
3. Destroy the pattern.
4. Load the pattern.
Should work fine.

5. Change the pattern.
6. Save the pattern using a button.
7. Load the pattern using a button.
Well, you should see the old pattern.
It didn't save it anywhere.

8. Change the pattern again.
9. Save the pattern using a redstoneblock.
10. Wait a second. (Important!)
11. Destroy the redstoneblock and the pattern.
12. Load the pattern using a button.
Seem to work. (As long as you don't re-enter the world.)