You're repainting a room in an old house. You're in a hurry, no time to do a thorough prep job. You think to yourself, "No big deal, I'll just paint over this old wallpaper", or "I don't know if this is oil-based or latex-based paint on the trim, but I'm sure the latex paint I bought will cover it just fine."
STOP! Don't do it!
You may be saving yourself time at the moment, but later on, someone will pay for this choice. You're handing the job of scraping off the paint-over-wallpaper, latex-over-oil paint to future generations, just because you were too lazy or rushed or ignorant to do the job right the first time.
Don't skimp on the proper preparations. People like me will be cursing you as we peel the paint off in sheets to do the repainting right.
This has been a Public Service Announcement, paid for by the hours of tedious labor I have put in as a result of my overactive conscientiousness.