Rogue users messing with their site – what happens to everyone else's site?

Hi

Basic question, lets say I have 345 users all working on sites that were all cloned and setup the same – theme, tweaks, plugins, settings.

Then along comes user #233 and she changes something, like hardcodes her site’s footer with a bunch of ads in it.

Does this go crazy and all of a sudden appear on everyone else’s site in the footer? – remembering they all use the same theme.

I’m new to this so apologies if this is the most basic question ever!

Thanks in advance.