depends on how trustworthy you think that the source of this certificate is.
if it is an company internal certificate -> you should at least inform the managing department of this certificate
if you look tat the certificate in detail and everithing else is as expected, you may continue
if it is an external company's certificate -> you should first verify that this company still owns the site where this certificate is used
be aware if a company has gone bankrupt, the domain-name may not have been closed (yet)
if it is a physical device (like web-config page of a network printer, or a soho router)
you may need to update it's firmware (containing a new certificate) first,
