It's like with all those bombs ever dropped, like e.g. cluster bombs.
The only sure thing is (as a US general put it) that they reach the ground
Some bombs are especially 'smart', as they were designed to explode an hour or two later, so the firefighters and civilians trying to rebuild from the rubble would be killed after the raid. The 'blockbuster' bombs designed to collapse whole house blocks of cities dropped over Germany also were 'smart', some never exploded due to failure ('Blindgaenger')
And some bombs were designed to explode when trying to defuse them - some nice chapters in the 'english patient', about that.
B.t.w. there still
are some nuclear missiles missing, that have been lost by submarines and planes, mostly including . And i do not want to know what the former soviet union 'lost' ..