I doubt that many people ever knew what the additional buttons on a modern mouse did until fairly recently.
In the days when the default button setting was ‘Forwards – Backwards’, not a great deal would happen if one accidentally pressed either of them.
These days, the default setting for the ‘Forward’ button is the Windows Magnifier. So, if the Windows Magnifier pops up out of the blue, it is not a Windows bug, it is not a virus, it is not a Trojan, it is not a registry corruption, ok? You have accidentally pressed the front button of the pairing.
To save you the time taken to ask in a newsgroup or forum on how to edit the registry with a view to making the Magnifier go away, there is an easier way.
PRESS THE BUTTON ONE MORE TIME..
The default setting can be changed in Control Panel’s mouse hole, but before you do this, consider that the Magnifier, now you have found it, is actually quite a useful feature sometimes, especially when reading EULA small print..