this is mostly for ddewaele, but also for everybody interested in this thread.
From the moment we got reports about this problem, we went into checking:
a) whether we could replicate the situation
b) whether we could fix alternative designs based on the feedback we are getting from all of you guys
This is what happened.
Gianluca, Arduino's HW mastermind, went into checking with 30 GSM shields in parallel for hours to see if any of them caught fire. The result was negative.
Then he started trying out different scenarios of possible misconnections and the one in the attached picture is the only one that made the capacitor blow.
Gianluca did as well try to rise the voltage on the shield up to 12V and that didn't get the capacitors to catch fire.
That said ...
1) the fact that we didn't manage to replicate the event it doesn't mean it didn't happen. Therefore, we are really sorry for the incident and we invite ddewaele to send me an email to try to compensate him/her for the incident: d DOT cuartielles at arduino.cc
2) we take your feedback really seriously, however, there is no capacitors of the size we need and a higher voltage in the right format. Since we might have to change the design of the shield all together, this is what we are thinking as possibilities:
2.1) putting two smaller caps in parallel, but with a higher voltage rate
2.2) putting the capacitor after the regulator, it is 4,85 Volts we need to take care of, so the restrictions are different, it is also good because the input will then take up to 20 Volts
2.3) adding a current protection (polyfuse or similar), Gianluca is looking into this
Summarizing, we will look at this and make sure we make it good.
Thanks for the feedback and sorry it took us a couple of days to answer. It is crazy with all the Maker Faires and conferences at this time of the year (besides it is school time and some of us teach/study).
Update: after posting this, Gianluca sent me an email with these two extra comments:
1) another option to let it explode is short circuiting Vin with 5V but it explodes only if Vin is>12V
2) For the next release [...] will add an extra zener for protection and also a polyfuse