Spectator players have the ability to edit beacon effects, though they should not.

A spectator cannot place the required iron/gold/emerald/diamond piece into the beacon GUI, but if one has already been placed into it (e.g. by a hopper or player) the spectator can easily select a different effect from any of the enabled tiers and click 'Done', which will update the effect and affect players in any other gamemode. This should probably be changed, and doing so only requires a check that gamemode != 3 before enabling any of the GuiBeacon's PowerButtons.

To reproduce:

• Load any cheats-enabled Minecraft world
• Build a one-tier or higher beacon (3x3 or greater)
• Place an iron ingot into the beacon, but do not click 'Done' (rather, click 'Cancel')
• Go into spectator mode with /gamemode sp
• Reopen the beacon interface (Note how you can select any of the effects available to other gamemodes)
• Select any of the available beacon effects, and click 'Done'
• Return to your previous gamemode and open up your inventory screen
• The beacon effect that you set in spectator mode should appear in your Status Effects list, indicating the changes were applied