Five warning signs a tech start-up is in trouble
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1) Key people start to leave.
2) The company loses focus.
3) The technology is great, but no one is using it.
4) There are public disagreements between the founders and the VCs.
5) There are competitors that have more buzz (and an audience).
0.5) The growth rate of customers is linear.
Of course, if you're in the "hype" phase rather than trying to make any money, those issues may not be that high a priority.