Here is a simple mistake I have made a couple times while wiring up a 555 timer. I have mistakenly wired the pins 5-8 side in reverse so instead of pin 8 across from pin 1 I have thought of that as pin 5 reversing the pin order on that side of the 555.
My old digital electronics prof. taught us if ever in doubt, start in the upper left and corner and count counterclockwise until you reach the upper right corner...
