90 day warning dialog box on ssl cert

About five years ago I was working for a company that somehow screwed up their ssl cert in such a way that, for 90 days, users' browsers would throw up a dialog window warning, saying that something or other was out of whack. Does anyone know what causes that?

I'm about to get a new cert for my site rather than upgrade my existing one, and I just don't want to screw something up that would cause that. thanks

 

 

 

 

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