Black powder weapons have many potential points of failure. The match or flint can fail to ignite the priming powder, the priming powder can fail to ignite the main charge. Those are both assuming that the weapon is perfectly loaded. In my experience it's easy to forget to load the powder before the bullet when distracted. Balls also frequently get stuck partway down the barrel as the fouling builds up if you use a ball that is too close in size to the diameter of the bore- a few hundreths of an inch make a massive difference. A ball stuck halfway down the barrel can't be fired or it is likely to burst the barrel and must be retrieved with a corkscrew, a fairly time consuming process.

Explosions were common, both of the weapons themselves and the powder. The explorer Captain John Smith was seriously burned when a comrade dropped a lit match on his powder bag while he was sleeping. During a skirmish outside Leith in the 1560's, a barrel of gunpowder which was brought up to resupply the skirmishers exploded and injured over twenty men. Weapons could burst when inexperienced men loaded too much powder, or the weapon overheated from being shot too fast, or the weapon was loaded with multiple bullets (a common practice in the 16th and 17th centuries). This could result in injured hands or eyes. An English captain in Ireland wrote home in 1601 complaining that the weapons his soldiers were using were so poorly made that five or six men lost their hands from muskets bursting in only six days.

These problems do diminish significantly with experience. Contrary to the common myth, musketeers did not replace bowmen because they were easy and quick to train. Musketeers received more, and more expensive, training than other sorts of infantry. They were expected to be skillful, celeritous, even "perfect" with their weapons.

Thomas Digges, during his time as muster-master general, advocated that musketeers should practice with their weapons daily. Unskillful men, the common opinion was, were more dangerous to themselves, their leaders and their comrades than to their enemies. This sentiment is ubiquitous among 16th-17th century military manuals. For example, Kellie Thomas wrote that "the musquet, as all fierie wapons, is dangerous to them who are Unskilfull, for an unexpert man may spoile himselfe and many about him". Another, Barnabe Riche, a soldier of such long experience that he lived to be the oldest captain in the entire Engilsh military, put it this way:

"What gaudes be these that you have inferred against shot? That they may neglect their sightes, that their bullets may be too low, that there may be fault in their pouder, that there may be fault in their match, that there may be fault in their charging: what objections be these but gaudes and trifles? For who would allowe him for a shot, that taketh no sight of his marke but shooteth at randon, or that caerth not whether his pouder be wet or dry, or that will not keepe his peece in servicable sort from rusting or furring, or that knowes not to give his peece her due charge, but will put in the bullet before the pouder, or that thinketh his peece never to be charged till hee hath filled the barrell to the very toppe? I have knowne such, but what of this? Are these things to be reckoned in the disparagement of perfect shot? No, they are but cavils, and the question is not of the imbicilitie of the man, but of the goodnes and advantage of the weapon being used in his kinde, and according to skill: But for these accidents before spoken of, if any of them happen, the fault is not to be imputed to the weapon, but to the souldier that hath committed some negligence. Confesse now a truth, and say that the musket & caliver are of greater force and service in the field or elsewhere, then any other shot that hath bin hitherto knowen, if the souldier that beareth them be expert as he shuld be, and neglecteth no part of his dutie."