When I read threads such as these, I am amazed that there aren't 10 times more of the severe 'flame'-related accidents in the general news.
Cells do generally not explode. Here are some of the test a cell must handle (From a Samsung datasheet):
9. Safety
9.1 Overcharge Test
Test method: To charge the standard charged cell with 12V and 2.8A at 25℃
for 2.5 hours.
Criteria: No fire, and no explosion.
9.2 External Short-circuit Test
Test method: To short-circuit the standard charged cell by connecting positive and
negative terminal by less than 50mΩ wire for 3hours.
Criteria: No fire, and no explosion.
9.3 Reverse Charge Test
Test method: To charge the standard charged cell with charge current 2.8A
By –12V for 2.5 hours.
Criteria: No fire, and no explosion.
9.4 Heating Test
Test method: To heat up the standard charged cell at heating rate 5℃ per minute up to
130℃ and keep the cell in oven for 60 minutes.
Criteria: No fire, and no explosion.
For the above test the cell will most likely vent.
HKJ, was it you who posted the 18650 disassembly pics that had mini-cells inside, or dirt/flour/etc ?
No, I have not succeeded in getting my hands on some of these cells.