One other thing is I do not believe the foam on the glass could ever cause your failure. It would compress under the weight, it`s just too soft to cause damage.
from what I understand, the compression is the problem.
Glass tanks with rims are designed to sit directly on the stand, with nothing inbetween the tank and the stand.
if the foam compresses too much (and touches the bottom glass) it puts upward force on the bottom glass and rim and can cause the bottom glass to crack.
this from what I understand is why tank manufacturers (eg. Aqueon and Marineland) do not recommend foam.
there have been long debates on this on here (of which I have been a part of LOL) and people have emailed Aqueon (aka All Glass Aquariums) and their response about foam was 'it can cause an uneven edge and cause the bottom to crack'.
Marineland simply voids the warranty on their tanks if you use foam.
so you have to be carefull when using foam. It can apparently cause more harm then good.
some like yourself have used it forever and had no issues. Others have had tanks crack supposedly due to foam used in the wrong scenario (should only be used for tanks with no bottom rim or acrylic tanks).
the hard part is trying to pinpoint why a tank failed. eg. it could be a glass defect and nothing to do with tank setup at all.
so sorry to hear abou the OP's tank. worst nightmare.
but pick up the peices and continue with the hobby. its still awesome