On a job interview listen carefully what the team you are supposed to join struggles with. What kind of problem they're currently solving.
Then if they say no to you, you can just say thanks and tell them
Hey, I'm good at this one you're dealing with, let me do this for you as a side project, show me the code and I will estimate it.
I suck. I totally suck at job interviews. Maybe I'm smart (oh the hell with it it's my blog, I can even say I'm very smart!). But I don't talk much, I'm kind of a typical introvert programmer. And as you may already know it's better to talk at job interviews.
So these guys were looking for someone who talks. They said
Sorry Piotr, you are very smart but you didn't say much at the interview, you had no energy and stuff. We're looking for a guy who can talk and has the energy.
And then I said
Thanks for the interview! Listen guys, you said you're struggling with upgrading your app. I did this many times and I can help you just with this one if you want. Just let me know if that sounds interesting.
It turned out this did sound interesting to them, we've discussed the details and I've got a nice side project instead.