Hi @Conny,
unfortunately this report is not very useful because it does not describe the problem well. If you have time and can still reproduce the problem, please add a more useful and complete description to this report.
What does "clicking tab left over (article)" mean exactly? Which browser is this about?
Why do you expect the system to warn you (does it sometimes do that)?

While in editing mode I clicked the article tab upper left. Normaly in MediaWiki editing mode Browser says, that there is unsaved content and asks, do I really want to leave.

User Agent: Mozilla/5.0 (Windows NT 5.1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/41.0.2272.118 Safari/537.36

If I click on tab "Lesen" there comes a message :) :

Bist du sicher?
Bist du sicher, dass du zum Ansichtsmodus wechseln möchtest, ohne vorher zu speichern?

I get a message when I click on "Artikel", but it's a different message coming from the browser. Works for me in both Firefox and Chrome.

Note that these messages only appear if you've actually made a change; if you try to leave the editor without having made any changes, it doesn't warn you.

@Catrope tested again, same problem. But this Chromeversion is old (because of the system here WinXP...).

In T96344#1216034, @Conny wrote:

@Catrope tested again, same problem. But this Chromeversion is old (because of the system here WinXP...).

Then it may well be a bug in your old version of Chrome. Have you encountered this bug with an up-to-date version of Chrome?