There has been a systematic search for UXO in West Berlin since the end of WW2 up to the late 70s, when it was considered that most of the stuff had been found and the rest was supposed to be rendered harmless by age.
Unfortunately there have been several incidents, first the discovery of a former anti tank trench in the south of West Berlin, where actually houses have been built on top, which was found to be full of UXO, in particular old anti tank "Riegelminen" (these mines, even if never armed, are considered especially dangerous, because, once the safety pin is pulled, they just rely on a thin shear wire to keep them from exploding, and the shear wire is often rusted away, the same happens to the safety pin, which once removed, can´t be reinstalled. They have to be blown up in situ).
Another one was the explosion of an American 1000 lbs bomb under a residential road. The bomb had a long time delay detonator, these bombs were dropped together with other bombs during air raids and were supposed to explode hours to days after the raid to disturb rescue and rebuilding operations. The detonator consisted of a stack of celluloid disks, which held back a springloaded firing pin, on dropping the bomb, a wire lanyard fixed to the plane pulled out a pin from the tail mounted fuze, which held back a small propellor. The propellor would turn in the slipstream and screw an arming screw into the fuze, crushing a glass vial filled with acetone. The acetone would then slowly disolve the celluloid disks until the firing pin got released and struck the primer, setting off the bomb. Due to some reasons (maybe due to the position of the bomb in the ground only acetone vapours reached the celluloid disks), the delay took 35+ years. Fortunately the explosion happened very early on a sunday morning with empty streets, only one woman had to be hospitalised with shock, though lots of windows got damaged.
These bomb fuzes were also booby trapped to prevent armourers from removing them (the American version has to be blown up on site, while there is a chance of disarming the British version with a powder turbine attached to it and fired from cover. The powder turbine in theory unscrews the fuze faster than the striker can hit the primer).
The last serious accident happened after the wall came down on a building site in Lichtenberg, East Berlin.
Builders were working on the foundations of a new building. Since Berlin soil is mostly sand, the standard practice is to drill holes of app. 3 feet diameter up to 20 yards into the ground and then, after putting a cage of reenforcing steel in, filling the hole with concrete. Somehow the drill must have struck the impact fuze of an American 500 lbs bomb (determined by police armourers from fragments found afterwards). The bomb exploded, killing three builders, injuring several others, partially destroying a neighbouring house and blowing in the roofs and windows of several other houses.
At about the same time the American and British governments released their old areal pictures taken after air raids, which show exactly where bombs, also duds landed.
This resulted in a new systematic search being started (as a college student I worked for one of the UXO removal companies for a while, the park I mentioned is the Tiergarten, we did a systematic search of this park, which is in the direct neighbourhood of Hitler´s former headquarters. We didn´t find any big bombs, just some of the hexagonal 2 lbs incendary sticks, but lots of 3,7 cm AA
rounds, small arms ammo, grenades, rusted SMGs and rifles, plus a few mortar rounds, from all kinds of countries used during the last days of WW2 in Europe. We also found an old aircraft radial engine from an American bomber. Since we didn´t have any hoisting equipment big enough we left it in the ground).
Also, according to law passed after the accident where the builders were killed, before the start of any building project in either Berlin or the state of Brandenburg, there has to be a search of the ground of the building site.
People back then had the nasty habit of just dumping unneeded ammo into bomb craters and then filling them up.
Hope this helps,