batteries will break open and leak acid when they are 'beyond dead' (so to speak).
a sure fire way to make this happen, is to mix 'new' batteries with 'old' ones. (mix 'strong' batteries with 'weak' ones)
for example if you mix two batteries, lets say: one half dead and one brand new,
the oldest/weakest one will of course "die" before the newer one.
but the overall circuit will still provide current as long as the "strong" one still has some life in it.
and this will cause the older ("weak") battery to crack open and leak. every time.
it can even happen over time, if you just leave the saber for a few months and don't play with it.
eventually the batteries will discharge on their own, the weak ones before the strong ones,
and the one(s) that discharges first (while the others still have a bit of charge) will crack open. every time.
(generally this doesnt usually happen if the batteries are all the same brand, all installed at the same time,
and all with the same amount of charge left in them when they finally "die")
---> I would take Kit's warning one step further and say, you should never mix&match batteries in ANY device. :-\