Faking topspin and backspin serves
Anyone who has played me knows I like to serve forehand pendulum sidespin-topspin serves that look like backspin. However, I less frequently serve backspin serves that look like topspin. (Instead I tend to mess opponents up by mixing in backspin and no-spin serves.) This has probably been a mistake - I should have developed those serves just as much, and recently I've reincorporated those serves into my game, as recent opponents have lamented. But why was I hesitant before?
First, a short note on how to do these serves - and keep in mind you can't learn them just from reading about them, you need to see a coach or top player do them, and perhaps get some coaching. (Here's an article on using semi-circular motion to disguise your serve. And in the video section here there are a number of videos showing top players serving.)
To serve sidespin-topspin and make it look like backspin, most of the semi-circular motion must be down, but right at contact the left side of the racket (for righties) snaps around, contacting the ball in a sideways and upward direction. Immediately after contact the racket continues down, and if the opponent doesn't watch carefully, it'll look like backspin. They push, and the ball pops up.
To serve sidespin-backspin, you essentially do the reverse. Right after the sidespin-backspin contact the racket rotates up, often with an exaggerated elbow motion. (Technically, an opponent could read these serves by assuming the spin is the reverse of the motion exaggerated, but you don't have time consciously read and react to a serve - it has to be reflex. Plus a good server keeps varying the motion, and the receiver can't pick up on the different motions quick enough.)