[asfm]

I have a problem with this puzzle. There seem to be at least three solutions. Now, I've already done it, so I don't need any help finding the answer, but I would like to know if there is a reason one solution is right and the others are wrong -- is there some hint I missed?

The correct answer is:








Al worked 6 hours - 12am - 6am (first shift)
Iggy worked 7 hours - 6am - 1pm (second shift)
Bernie worked 8 hours - 1pm - 9pm (third shift)
Pop worked 3 hours - 9pm - 12am (fourth shift)

That would make the time of the explosion 8pm.

But:

Al worked 6 hours - 12am - 6am (first shift)
Iggy worked 7 hours - 6am - 1pm (second shift)
Pop worked 3 hours - 1pm - 4pm (third shift)
Bernie worked 8 hours - 4pm - 12am (fourth shift)

That would put the explosion at 3pm. I believe there's also yet another solution, but a friend found it and I didn't make a note of it so I can't repeat it here.

Anyway, both of the above hold to all four rules. In both, Pop works three hours, Bernie works eight hours, Iggy worked from six until he was relieved (while working more than 3 and fewer than 8 hours), and the explosion was heard during the third shift.

The only possible explanation I can see for the first example being correct is that Bernie works the third shift, and it's Bernie who says 'BOOM! One hour before the last shift started', but that statement in no way affirms that it was Bernie who actually heard the explosion. This is also exemplified by the fact that Al makes a statement about Bernie's shift, meaning it stands to reason that Bernie's comment may also be about someone else's shift.

Am I missing something or is this puzzle broken?