Q: left click on mouse stops working when iphone plugged in to OSX

Hello (RoHo),

Thank you for using the Apple Support Communities!

It is my understanding you are experiencing intermittent left click issues with your mouse.  Being able to left click is an important part of using your mouse.  I have a couple suggestions that can help you.

It is important to try to isolate what is causing this to happen.  You mention that it seems to have something to do with plugging in your iPhone, but that it does not always happen when the iPhone is plugged in.  The first thing to do to help isolate what is causing the problem is to start the computer in what is called safe mode and see if the problems still occur.  

Try safe mode if your Mac doesn‘t finish starting up

Starting up in safe mode

Follow these steps to start up into safe mode.

1. Start or restart your Mac. 
2. Immediately after you hear the startup sound, press and hold the Shift key.
3. Release the Shift key when you see the Apple logo appear on the screen.

After the Apple logo appears, it might take longer than usual to reach the login screen or your desktop. This is because your Mac performs a directory check of your startup disk as part of safe mode.

Once the computer is running in safe mode, see if you can duplicate the issue.  If the issue does not appear, go ahead and try restarting normally again.  If the issue is gone, it may have been caused by a cache file or directory issue that starting in safe mode fixed.

If the issue returns, you are going to want to check login items.  You can find login items by clicking on the Apple Logo in the upper left corner, then click on System Preferences > Users & Groups.

Follow these instructions to remove the login items:

OS X El Capitan: If you think you have incompatible login items

Have a great day!