GTV-6, broken lugs, advice needed

[Joe Elliott <jee@xxxxxxxxxxxxxx>]

I noticed yesterday that two of the lugs on my right front wheel are 
broken off.  I haven't done anything weird or abusive, but I did 
install new brake discs (and lugs) about 1000 miles ago.

My questions are:
1) Why on earth did this happen?  (A Miata-owning aquaintance wasn't 
surprised and said he's seen this happen before, but I can't see what 
they cause could possibly be.)

2) How dangerous is it to drive (conservatively) with two of five 
lugs broken off on a hub-centric wheel?

3) Even if the answer to #2 is "pretty safe," what are the odds of 
additional lugs randomly breaking off over the next 500 miles?

Thanks,
Joe Elliott
'82 GTV-6
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