Minecraft wiki entry for Redstone_tips, associated text: Be careful when building circuits to activate TNT (traps, cannons, etc.). Circuits in mid-construction can sometimes briefly power up unexpectedly, which might activate TNT. For example, placing a redstone torch on a powered block, it won't "figure out" that it should be turned off until the next tick, and can briefly power another part of the circuit until then. Placing TNT after the rest of the circuit is complete will help to avoid such problems and the destruction of the device itself. This also applies to any other features of the circuit that may be accidentally activated with such actions (e.g., activating a dispenser before the circuit is ready). Temporarily placing a redstone lamp or piston can quickly test whether a given space is powered.