'07 Pontiac Solstice 2.4, which had an aftermarket tune.
I downloaded the tune. When I started the download, a bunch of relays clicked and clacked but settled down quickly. (I've never seen that happen through scanning; not sure what was going on there.)
Once the download was complete, the car refused to start. The DIC showed the message "Engine disabled."
We rebooted the car - ie, disconnected the battery, discharged residual charge, reconnected, still disabled. I scanned it with the VCM scanner app, didn't see any codes. I went ahead and cleared the (non-existent) codes. Then, the car started.
WTF? This was slightly nerve-wracking as I was nearly left in the position of having to pay for a 1300 mile tow.