An accident is a "mishap or untoward event not expected or designed", or "unforeseen contingency or occurrence" Its meaning varies depending on the type of charge. Where it is a specific intent offence, an accident relates to a denial of voluntariness of the act or denial of intention to cause the outcome. General intent offence claims the act was unexpected or chance that was not foreseeable. 
The effect in law is that the mens rea is not present. This is distinctive from mistake which occurs in “the realm of perception”.
Once the accused establishes there is an "air of reality" to the defence, the crown must disprove the availability of the defence. 
R v Whitehorne 2005 CanLII 34553 (NLPC)
Hill v. R., 1973 CanLII 36,  2 SCR 402, (1974) 14 CCC (2d) 505 (SCC) at p. 510
Criminal Pleading & Practice, Ewaschuck, (2nd Edition) at para. 21.0030
R. v. Mathisen, 2008 ONCA 747 at 70
- R v Whitehorne
- R. v. Sutherland, 1993 CanLII 6614 (SK CA), (1994) 84 CCC (3d) 484 (Sask. C.A.) per Vanscise, J.A., at pp. 497-498
It is permissible to argue that the action of accused was not voluntary as it was a reflexive act.
R. v. Pirozzi (1987), 34 C.C.C. (3d) 376 (Ont. C.A.)
R. v. Mullin (1990), 56 C.C.C. (3d) 476 (P.E.I.C.A.)
R. v. Wolfe, (1974) 20 CCC (2d) 382 (Ont. C.A.): accused hits victim on head with telephone by reflex