I also have a C-Tec brand charger as well. Yes, I did have an issue with this one about a month before the Battery Tender Junior blew up my battery in a different car.
The C-Tec was on my part time daily driver 89 Mustang and used in the driveway. Yes, the battery was probably old as it came with the car. In this case, it was found after a couple of days, seeing a puddle etching into the concrete. A quick wash down, remove the hood pad and a new battery, it was good to go. After the second incident, I gave it some more consideration. The C-Tec charger was still functional. I sent C-Tec an email and they responded immediately saying this should never happen and wanting more details. I never followed as I was busy trying to see what Battery Tender was going to do about the two show cars.
C-Tec is an option on many cars, tractors and motorcycles. I also like the feature to recondition the battery during charging.