With Android 6.0 Marshmallow, there are going to be a decent amount of small changes users will come in contact with, alongside some that are not so small. One of the changes users will want to be looking out for is the changes to permissions which is something which was discussed earlier on with one of the Android M developer previews. Permissions in Marshmallow will be a little more granular than before and instead of just asking for permission when you install an app, apps will request permissions upon trying to use the app. This makes it so that an app users have installed but may not touch for weeks won't have permissions it doesn't need right away. Users will also be able to toggle permissions on and off when they want.
Now that Android 6.0 Marshmallow is already out and starting to hit Nexus devices, users may be coming across some quirky little hiccups with applications not working right. The reason behind this seems to be that certain apps don't have required permissions enabled to utilize certain functions, although this isn't all apps once you're on Marshmallow and seems to be limited to those which have been updated target level 23, which is Android 6.0. One issue users might come across if they've already updated to Marshmallow is with Google+ when they try to create a new post. While the post opens up just fine and you can type up whatever it is you went in to post, the box of your most recent images might not be appearing below the text box and it's because the "storage" permission for Google+ is likely disabled.
Simply going into your apps list from the settings menu, finding Google+ and tapping it then selecting Google+ permissions, and finally enabling the storage permission toggle should fix the problem, and going forward your images should show up under the text field in new G+ posts so you can easily insert images. Users may also notice weird things happening with other apps where certain stuff isn't working quite right, and if something seems a little funny, the best practice would be to go and check all the permissions. If you aren't wanting to enable all of them, you could go down the line and toggle them one by one, then attempting to do whatever it is that wasn't working after each toggle to narrow things down. It'll take a bit longer, but you'll at least end up finding out the exact permission you to need to grant access to.