And reflect this in the bill
I had a project that required a 3-d video card. I did the research, and selected a card that looked perfect - but did not have a chance to try it out first. I get to the customer site, install it, and the available drivers will not put the card into 3-d. I used my prior research, and selected another card, called the vendor, and got a verification that it would work (the first vendor claimed the same until I was actually on-site - sigh). I went to the client and explained the situation, and how I was going to fix it with another card. He bought the second card, it worked, and was happy.
I gave him my invoice, and explicitly comp'd him the 3 hours it took for the second card. He was very happy.
So - be prepared to eat some time, but note it on the invoice to educate the client. It also can make you look very good - you are willing to take responsibility for your errors, even though you did your best.
