The gas company put the wrong gas in the cylinder you fill from?

The wrong grade of helium is in them?

There are impurities in the gas?

You miscalculate your mixtures and have too much oxygen?

Too little oxygen?

Too much nitrogen?

Too much helium?

The gauge for mixing is wrong?

The filling temperatures are different?

The dive shop filling your tanks allows the gas to bleed back into their bank system?

Your cylinders contain flammable materials and you put in pure oxygen?

The dive shop tops your deep tanks with bad air?

A pinhole leak allows the helium to leak but other gases remain?

You get the cylinders mixed up while filling?

You don’t have enough oxygen when you submerge?

Your depth gauge reads shallow?

A tank you thought had trimix was really nitrox, etc.?

You overbreathe your regulator?

You have a delay before ascending?

You have a delay on the way down?

You have to exert a lot on the bottom?

You have a five minute line entanglement, etc., on the bottom?

You lose your dive partner on the bottom?

You get blown from the dive site?

You lose your mask?

The decompression tables you have don’t work?

You lose your decompression tables?

You lose your watch?

You lose your depth gauge?

You run out of decompression gas?

You drop one of your decompression cylinders?

Someone steals your decompression cylinders?

Your decompression cylinders have leaked?

You are delayed at a decompression stop?

Danger from hazardous marine life keeps you from completing your decompression?

Your dry suit floods?

You get hypothermia?

You get the bends?