Just about all ‘click to use’ items have some form of cool-down.

In addition, many items have a shared cool-down. This means that you cannot use a bunch of trinkets, gadgets and bombs in quick succession (Beside the 30 second wait time you see loong gold after equipping such an item.).

What Locks Out What?

Nearly all damaging engineering items lock each other out.

Let me give you the full list as an example; All Bombs, Dynamite, Goblin Sapper Charges, Goblin Mortar, Portable Bronze Mortar, the Gnomish Death Ray, Goblin Bomb Dispenser, Goblin Dragon Gun, Goblin Land Mine, Explosive Sheep, Gnomish Flame Turret, Discomboulator Ray and Compact Harvest Reaper Kit lock each other out for 1 minute.

Summoning a Target Dummy locks out other target dummies, Minor/Major Recombobulator and the Gnomish Alarm-O-Bot.

Summoned engineering pets are aggressive and will engage any nearby hostile targets, which can make them a liability at times.

Identical trinkets share the same cool-down. (Making two shrink rays will not allow you to use them in succession; they will both get the 5 minute cool-down after using one)

Chapter 5 has further information on trinkets and gadgets, including their shared cool-downs.