If I work at home this weekend to get some undersized stuff done by monday . Should I ask them to pay me extra?
Or we devs do not have those privileges?

I have the somple principle that if the problem comes from me being stupid, useless and shouldn't be allowed anywhere near a keyboard I will not bill anything because they already paid the failed implementation. If they change specs, want something new added on the last minute or decide to reinvent the whole project, they'll pay for it. TL;DR: I pay for my stupidity, they pay for theirs.