how so? (assuming normal conditions)
With most batteries you can get runaway reactions that lead to tremendous temperature increases and, if the electrolyte is flammable (never got used to the change from inflammable to flammable) or anode/cathode products combustible or gaseous then fire/explosion is almost inevitable. These can occur for a number of reasons - faulty manufacturing introducing inhomogeneity into electrodes, electrolyte impurity - the age of the battery itself can be responsible for physical degradation and side-reactions that cannot be entirely eliminated anyway, improper use/storage etc. With rechargeable batteries the charge/discharge cycle is important because degradation of electrodes results, particularly on overcharge which will affect the longevity of batteries. Some batteries have control circuits to limit or mitigate runaway reactions by limiting voltage but the presence of such features is not a guarantee of reliability or safety.
Of course, failure is rare but when it does happen it can result in serious injuries as the unfortunate lad in incognito's original post experienced.
If you're going to use a high power density cell, such as are used in mobile phones, don't overcharge them, keep them cool, replace the battery every couple of years and don't keep them in your pockets - particularly any pockets close to your family jewels.
