Summary of what happened:
Upon changing a Minecraft server running version 14w10c from online-mode=true to online-mode=false, all operators are unable to use any commands that are given explicitly to operators, such as /setblock or /gamemode. In addition, operators were unable to modify data in command blocks, since the server believed that they were not operators. On looking at the ops.json file, all users with operator status were correctly listed and had a "level" value of 4.

What I expected to happen was...:
The server should have changed from online-mode to offline-mode without a hitch and any users with operator status should have been able to use operator-only commands, such as /setblock or /summon.

What actually happened was...:
The server sent out several error messages on the console and any previous operators were unable to use operator-only commands.

Steps to reproduce this issue:
1) Download the latest server .jar file from the Mojang official website.
2) Run the server .jar file and let the config files be generated for the first time.
3) Stop the server.
4) Edit the server.properties file and change online-mode to false.
5) Restart the server.
6) Type /op [playername] to op any player.
7) The said player does not gain operator privileges and cannot use commands such as /summon or /setblock, which are only available to operators.

However:
This bug does not occur when the server is set to online mode. Players can gain operator status, whether via the console or by modifying the ops.json file, and the operators can use commands like normal. However, when changing the mode from online-mode to offline-mode, the players lose operator privileges, even if their username, UUID, or permission level remain unchanged in the ops.json file.