So you take a contract where you have solid experience with 75% of the technology necessary. How do you handle your time to learn the other 25%?
Work it into the billing time? Expose the 25% in the contract as 'research'? Do the learning on my own time (not billed)? Not take the contract (too large of an unknown for me and the customer)?
On the extreme end of this, I keep hearing a story about Mark Cuban (Dallas billionaire who started broadcast.com and sold it to Yahoo!) when he was at Indiana University. Someone asked him if he could build a business app for them and he immediately said "Yes"... he had no idea how. So he bought a book, stayed up nights, studied and coded... He finished it (I'm sure it was ugly), it worked and he kept going.
I'm not suggesting doing contracts this way (the stress!), but there's a middle ground. What is it, and how would you (or would you?) bill for the unknown?