One that happens frequently.
- Go into Photos and select one or more pictures.
- Select the Action button and share either via Messages or Mail.
- Start typing in the email address or phone number to share.
- At some point during this process (in Messages mine is usually when I select the person from the popup list in messages as I type the phone number) the keyboard will vanish.
This does not happen all the time however it happens frequently.
There are a number of other way however this is the most frequent fail for me. I also sometimes have the screen freeze mid-type.