I've created a test that click on an element X.
This element is only revealed after you click on another button,
and those elements are connected with ng-hide.
When i try to run my code the click on X element doesn't work.
However, in debug mode or after adding 1 second sleep, it does.  
I'm using selenium framework in python, with a remote webdriver with ImplicitlyWait of 10 sec.
Does someone knows the reason for this behavior?
As said by @Siking, this is clearly a timing issue.
The fact is that Selenium is very fast and faster than loading of element.
Sometimes, Selenium requires a pause or a sleep to ensure that an element is present.
I also recommend -especially for asynchronous requests- using waitForElementPresent to wait until ajax method is finished.
Selenium click on elements works only in debug mode