Recently four Ukrainian sailors were seriously injured when two 30mm cannon shells spontaneously exploded. Actually, those shells didn't go off entirely without warning. The navy reported that the shells were old, beyond their use by date, and were probably set off by vibrations ships generate during training exercises.
This is a common problem in countries that have long used ammunition bought from Russia. During the communist period, as per the Soviet custom, old ammunition was not destroyed, but kept around. Communist countries were poor. It made sense to keep those old mortar and artillery shells (plus bombs and military explosives). But the chemical reactions taking place in propellants and explosives, once these items are manufactured, eventually cause dangerous side effects. Over time, the compounds, that make the propellants and explosives work, deteriorate. This renders the propellants and explosives useless or, in many cases, unstable and very dangerous.
This has resulted in many spontaneous explosions on Russian ships and in ammunition depots. These accidents also happen outside Russia. Two years ago, an Albanian ammunition processing facility north of the capital exploded. There were over 200 casualties, including at least nine dead (largely among the 4,000 civilians living nearby). Over 300 buildings were destroyed, and over 2,000 damaged. The facility was used to destroy old ammo, which is a condition for Albania to be allowed to join NATO. There were about 100,000 tons of old ammunition in Albania, and the destroyed facility dismantled 500-600 tons of the stuff each month.
The explosion in Albania probably occurred during the process of extracting explosives from the old ammo. This can be tricky, as the least little spark, can set this stuff off. Worse, older ammo in an unstable state can go off without a spark.
This disaster was part of a trend. Three years ago, there was a large explosion in an ammo depot in the African nation of Mozambique. About a hundred died. Six years ago, an even greater disaster occurred in Nigeria, when a munitions depot near the capital cooked off, killing over 200 people.
Russia has also had problems with elderly, and cranky, munitions. In the 1990s, there were several munitions depot explosions, some of them quite spectacular. Russia, however, tended to put these depots in isolated areas, so the casualties were low. However, the Russians took the hint, and disposed of huge quantities of Cold War surplus munitions.