I cannot claim credit for this, it's nicked from a link that Breezer posted elsewhere, but made me smile anyway: 
The Self-Uninstalling Gas Water Heater
By Andrew Gabriel
On Wednesday, my trusty Main Medina multipoint gas water heater
decided it was time to depart from this world. Age unknown, but
I guess at some 20-25 years old, it's filled a fair few baths
and handled many showers in its time. Anyway, my Wednesday
morning's bath was to be its last. As usual, it did a great
job of providing a nice hot bath of water. However, it seems
that it had a momentary lapse of concentration, that is, it
failed to notice I turned off the bath tap and that the flow of
water through it had stopped. Merrily, it continued to pump
33kW of heat into the pint or so of stationary water in its heat
exchanger. Well, it didn't stay either stationary or water
for longer than a few seconds. What with senility having set in
far enough that it had forgotten what its role in life was, and
with having found that it could make steam at a rate and at a
temperature that Stephenson and Watt would have been truly proud
of, it duly embarked on its final mission, to uninstall itself.
Steam production only within the confines of its own pipework
was never going to be very satisfying exercise by itself - it
would be much more exciting to involve all the household
plumbing. There's the little matter of the flow restrictor
valve on the water inlet which could limit the rate steam can
be pumped out, but since that's only got a plastic centre,
suitably hot steam can just melt it out of the way, so that
problem is easily overcome. So now let's see how far back up
the water main we can blow steam - quite some way it seams,
certainly far enough that a very respectable jet can be ejected
from any cold tap which someone might happen to turn on. This
gets boring after a while - have to find something else to do.
Ah yes, get the steam hot enough and under enough pressure, and
the solder in all the pipe joins/elbows can be melted and the
joints blown apart - now there's a good laugh.
Well, by this stage I'd realised the house plumbing was having
a fight with something, and the water heater was starting to
let off a bit of a hot smell, so I quickly turned it off, just
before any joints had completely separated. This was followed
by cold water coming back into the hot pipes, which reset all
the solder joints (not how they were before). It then picked
up the molten guts of the flow restrictor valve and transferred
it back into the water heater, leaving it to solidify in the
flow detector pipe constriction, thereby completely blocking
the water path through the heater.
Anyway, after a minute or two of surveying the situation and
realising I had no further hot water (in fact, not any water out
of the hot taps because the heater water path blocked), I decided
to go and make the most of the last hot bath I might be getting
for a while, which I'd just finished running.
Upon emerging from the bath, a post mortem of the heater and
pipework ensued. It was decided against any resuscitation
attempts, and death was pronounced.
The Self-Uninstalling Gas Water Heater
By Andrew Gabriel
On Wednesday, my trusty Main Medina multipoint gas water heater
decided it was time to depart from this world. Age unknown, but
I guess at some 20-25 years old, it's filled a fair few baths
and handled many showers in its time. Anyway, my Wednesday
morning's bath was to be its last. As usual, it did a great
job of providing a nice hot bath of water. However, it seems
that it had a momentary lapse of concentration, that is, it
failed to notice I turned off the bath tap and that the flow of
water through it had stopped. Merrily, it continued to pump
33kW of heat into the pint or so of stationary water in its heat
exchanger. Well, it didn't stay either stationary or water
for longer than a few seconds. What with senility having set in
far enough that it had forgotten what its role in life was, and
with having found that it could make steam at a rate and at a
temperature that Stephenson and Watt would have been truly proud
of, it duly embarked on its final mission, to uninstall itself.
Steam production only within the confines of its own pipework
was never going to be very satisfying exercise by itself - it
would be much more exciting to involve all the household
plumbing. There's the little matter of the flow restrictor
valve on the water inlet which could limit the rate steam can
be pumped out, but since that's only got a plastic centre,
suitably hot steam can just melt it out of the way, so that
problem is easily overcome. So now let's see how far back up
the water main we can blow steam - quite some way it seams,
certainly far enough that a very respectable jet can be ejected
from any cold tap which someone might happen to turn on. This
gets boring after a while - have to find something else to do.
Ah yes, get the steam hot enough and under enough pressure, and
the solder in all the pipe joins/elbows can be melted and the
joints blown apart - now there's a good laugh.
Well, by this stage I'd realised the house plumbing was having
a fight with something, and the water heater was starting to
let off a bit of a hot smell, so I quickly turned it off, just
before any joints had completely separated. This was followed
by cold water coming back into the hot pipes, which reset all
the solder joints (not how they were before). It then picked
up the molten guts of the flow restrictor valve and transferred
it back into the water heater, leaving it to solidify in the
flow detector pipe constriction, thereby completely blocking
the water path through the heater.
Anyway, after a minute or two of surveying the situation and
realising I had no further hot water (in fact, not any water out
of the hot taps because the heater water path blocked), I decided
to go and make the most of the last hot bath I might be getting
for a while, which I'd just finished running.
Upon emerging from the bath, a post mortem of the heater and
pipework ensued. It was decided against any resuscitation
attempts, and death was pronounced.