Since a few days, my Mac OS wasn't running well.
Strangely, it began a some time after updating through Mac OS software updates. The symptons were a black frame on selected items like icons, links, etc...; each item of Apple Menu duplicated and safari was always crashing while opening mails on Gmail website .

This evening, I've turned the sound on to hear a video. Then, I was surprised to hear a voice which was saying all I was doing and describing each window. In fact it was the accessibility tool of Mac OS named voiceover[1]. I certainly have activated it by keying unconsciously Command[2] + F5. Juste type same keys to turn it off and every problem had disapeared[3]!

Notes

[1] Big brother is watching you and knows about all your moves! All your desktops are belong to us! <eg>

[2] "cmd" key

[3] Back to the normal (what is normal anyway, nowadays ?)