Issue with the fn-Button and the F1-F12 buttons

Hi guys.

I have had the following issue unreguarly since I bought my MacBook 13,3" unibody on Wednesday.

Sometimes the function F1-F12 buttons are "switched around". That means, I don't have to hold the fn-button to execute the function of the F-buttons.

I already found out that this can be changed in the System-Preferences in "Keyboard & Mouse". If I turn the function on everything works the way I want it to. But sometimes it kinda automatically switches this option off.

I hope you understand my problem.

Is there a shortcut to switch that option on/off maybe and I use it unwittingly? Any suggestions? Till today, I am unable to reproduce the issue because it's not regular.

Thank you so far.

I am writing from Germany and I have to admit: it's been a while since I used my English last time. But for all that I hope, everything can be understood. If the phrasing doesn't sound familiar to you, don't punish me, please