What issues do invalid certificates cause for Mac's and for Windows based pc's?
If you're talking about "self-signed" certificates that computer can generate, they don't really cause any issue. You can either accept them or not. They do not add or remove functionality of the client or the server.
Within your own controlled network environment, you probably can accept them without much fear, since it's your own network.
If you're connecting to a machine you do not know (e.g. web server, file server) that you don't know on another network. Then it's better if you do not trust this certificate they have because they cannot be verified either by you or a trusted 3rd party.
Remember that certificates are really about identification. For example, a Government Issued ID similar to a drivers license for me. If you ask me for a valid ID and I make one to give to you; you can accept it. The issue here is the only one who can vouch for my identity is me.
But if I give you my driver's license. You can choose to accept that ID that was issued to me by the government. This mean that someone besides me is also vouching for my identity.
Let me know if this wasn't your question.
thank you Ronnie
If you would like to see the results of different types of invalidate certificates you can browse to https://badssl.com/
which is a site that I use on the show to demonstrate certificate valid. It is from some of the Google and Mozilla dev crew. Hopefully you find it valuable too.