I've strange problem.

Line 61: $this->_currentRoute = Default_Model_Routes::getInstance()->getCurrentRoute();
.......... other code ..........
Line 86: var_dump(isset($this->_currentRoute['url']));
Line 87: var_dump($this->_currentRoute['url']);
Line 88: if ($this->_currentRoute['url'] == $currentUrl)
Line 89:     $navigation[$key]['active'] = true;
Line 90: var_dump($this->_currentRoute);

This really is result:

string(62) "cs/Polozka-menu-1/Polozka-menu-1-1/Polozka-menu-1-1-1/Clanek-1"
array(17) {
  string(62) "cs/Polozka-menu-1/Polozka-menu-1-1/Polozka-menu-1-1-1/Clanek-1"
  string(7) "article"

As well as in the mistake log:

[09-Mar-2011 19:49:32] PHP Notice:  Undefined index: url in ...file... on line 87
[09-Mar-2011 19:49:32] PHP Notice:  Undefined index: url in ...file... on line 88

Please, for those who have any ideas where might be problem or how you can fix, let me know. Thanks :)

I've attempted another test:

$test = array();
echo $test['lol'];

With this particular result:

Notice: Undefined index: lol in ...file... on line 92

Somethink interesting: THIS 'lol' error is displayed in output, However the 'url' error Isn't ! It is simply within the log ... how come ????? It's same file, line underneath the 'url' var_dump() ... crazy

What goes on should you assign $this->_currentRoute to some variable first?

$route = $this->_currentRoute;
var_dump(array_key_exists('url', $route));
print "PRINTING: ".$route['url'];
die("ENDING ON LINE [".__LINE__."] !!!");

Solved (see comments)

It was regrettably an problem with Zend Server, or bleeding edge PHP 5.3.3.

Maybe this isn't you errors?
Attempt to ini_set('display_errors', 1);

As well as see method Default_Model_Routes::getInstance()->getCurrentRoute()::__get()

Look at this