Sometimes I found only a small $.50 vacum hose that rotted off or saturated with oil and just fell off! The shop Managers always tell us to fix the small problem but upsell somthing else. Which brings up another Idea they have.
Somtimes, it only takes less then 20 seconds to fix a problem but they have to hold the car in the shop for at least 30 minutes to make you feel like your money was well spent. Beware of this tactic also.
Like I said before not all shops act this way but you would be surprised of how many perform this tactic on the unsuspecting Customers.
And last, If your check engine light comes on, just ask for a diagnostics of why your light is on. Many shops can go into an hourly charge when it comes to a check engine or service engine soon light. But just ask for a diagnostics check.