My experiences with tantal capacitors is the same as one of the
above posters, i.e. the thing that happen when they go defective is
that they behave as a very efficient short-circuits. If then there is
enough current capacity of the power applied to this
short-circuited defective tantal capacitor - you'll get a black
bad-smelling little piece left.
A few of the manufacturers of recent types of tantal capacitors
are though claiming that they have managed to come to terms with
this "normal" failure mode of their brand of tantal cap's. My advise
would be to search and investigate some of the manufacturer's
data-sheets regarding their most recent lines of tantal capacitors
and see which of them that claims that their capacitors do not go
into short-circuit as the normal failure mode.
Also, when there is any risk of the voltage being applied for some
moments with the wrong polarity to the capacitor, connect a diode
across it to bypass the current. Tantal cap's are, as I have noticed
by practise, very sensitive to the voltage being applied in the
wrong direction. You can also try to exchange some of the
tantal capacitors to high-capacitance ceramic types (they do have
a long life-time and no requirements on polarity).