Ambiguity in contracts is a major pain in the ass. To the employee a "reasonable amount" of unpaid overtime could well be ZERO. I'd certainly want to be paid for it. But to the employer it might be an extra 40 hours a week, which to me is ridiculous.
The only people in the company that should be working unpaid overtime are the founders, because its their company. If you're an employee, you should do your work that you're hired for and then go home.
Really, I had this in my contracts a lot as well (Germany).
I tend to accept it if it's rare (like - release work with some pressure, for a limited time). But - it was a learning curve for me.
When I started my first 'real' job I was constantly working overtime (10-12 hours, daily), no compensation (and a low salary). I sold myself far too low - but at that time it didn't occur to me. Evil team dynamics can make this even harder: IF someone from our team decided that 22:00 is late enough and packed up, we'd half-jokingly complain about him being on half-time. In retrospect, this idiotic behavior just helped our employer to milk more time (not really work, I'd argue.. Productivity goes down fast after a while) out of us.
My last contract again included this clause, but I grew up and learned to say no, leave if I'm tired and turn off my mobile. If you're no jerk about it I'd expect it to work flawlessly. Of course your employer would love to see you work more for less/the same. And he certainly is free to ask me to 'do this one thing'.
And I can always decline, and do so most of the time now.
Why does a contract ever provide for unpaid overtime?
Surely overtime is necessary once in a while to match a deadline. However, having a modest premium on it is an easy way of ensuring the amount of the overtime will remain reasonable.
It can be compensated differently. For example, it's pretty standard in Sweden to have 6 weeks of vacation (instead of the usual 5) in exchange for reasonable unpaid overtime.
The only people in the company that should be working unpaid overtime are the founders, because its their company. If you're an employee, you should do your work that you're hired for and then go home.