I've been bitten twice by using new, untested equipment, and I'll NEVER do it again. I used untested lenses on some shoots. I got away with it a few times with some lenses, but twice they were horrible. One back focussed quite badly, and the other (used lens) had fungus deep inside one of the center optics that couldn't bee seen without a very bright light. Of course the fungus completely ruined the contrast. Two ruined afternoons, and a great deal of embarrassment.
Fortunately the seller of the Nikkor with the fungus was an honest man, and immediately sent a refund, along with postage both ways. The other 3rd party used lens could not be returned, and I was so angry I went out that afternoon and bought a Nikkor replacement at full list price - the 28mm f1.4 (the week before they announced it being discontinued). A week later I heard about it being discontinued, and I was REALLY mad, thinking I just paid a fortune for an obsolete lens. I'm not too unhappy about that now .
I also check my CF cards by running a bunch of non important pictures on them before using them for anything important. So far, so good.