I have killed a few batteries with battery tenders. I've never heard of this happening to anyone else, ever, so I'm not too sure what went wrong.
Here's the long story of how it happened, if anyone can make sense of what went wrong be my guest. Skip over this if you don't want to read a story:
ZX-14, killed 3 batteries. Road King, killed 2. GSX-R 600, Aprilia Mille, and ATV killed one each. Every one of these instances the batteries were put on a tender at all times, and they didn't last over a year. The ninja was the worst, killed a battery every 6 to 8 months. I used the same tenders as my dad, actually we switched tenders a few times thinking that may be the problem. I quit using tenders and haven't had a battery problem since. He still uses them, and hasn't had a problem.
What I do now, is nothing. When I park the bike for the winter I act like they don't even have batteries. Next spring when it's time to ride I start them and ride away. The ATV gets ridden all winter, the rest of them just sit in the garage waiting for a warm day.