Keyboard...?

[Aerodyne]

Overtype (OVR?) occurs with email, so probably with browsers too. It is easy to inadvertently select it by a glancing touch to the wrong key so it may not be a fault as such. You might be contacting your 'insert' key. Try it when the fault occurs - one tap on the insert key to clear the overtype setting, another single tap on the insert key to set it up.
Hope this helps.

Tony