Outside Counsel

In my experience the admissibility of custom and practice evidence is poorly understood, and I expect that this is because the rule is incoherent-- or at least generally expressed badly.

in Halloran v. Virginia Chemicals,¹ a products liability case in which the defense sought to introduce evidence of plaintiff's "usage and practice" to use an immersion coil to heat the water into which the freon (the product) was placed, causing the explosion seriously injuring the plaintiff.
On cross-examination by defense counsel, the plaintiff, an automobile mechanic, denied ever doing this. The defense offered a witness prepared to testify that he not only saw plaintiff using the immersion coil to heat the freon on previous occasions, but also warned plaintiff of the danger as well. Plaintiff's objection to this proposed testimony was sustained by the trial judge relying on the well-settled rule that extrinsic evidence cannot be used to impeach a witness on collateral matters. The Second Department affirmed, and a question of law was certified for review.
The Court of Appeals, in reversing judgment for plaintiff and granting a new trial, held for the first time that habit evidence of carelessness or carefulness may be admissible under limited circumstances to prove the actor was negligent or not negligent on the occasion in question. In this case, if the auto mechanic had habitually or regularly used the immersion coil to heat water into which the refrigerant container was placed, evidence of that habit was admissible with a proper foundation to prove that plaintiff followed such a procedure on the day of the explosion, and that such evidence in this case was not collateral since it would explain the explosion, and therefore, did not violate the rule against using extrinsic evidence solely to impeach credibility on a collateral issue.
The court reasoned:

Evidence of habit or regular usage, if properly defined and therefore circumscribed, involves more than unpatterned occasional conduct, that is, conduct however frequent yet likely to vary from time to time depending upon the surrounding circumstances; it involves a repetitive pattern of conduct and therefore predictable and predictive conduct. On this view, the excluded evidence was offered to show a particular method of executing a task followed by the mechanic, who, on his own testimony, had serviced "hundreds" of air-conditioning units and used "thousands" of cans of the refrigerant. If on remittal the evidence tends to show that the mechanic used an immersion coil a sufficient number of times to warrant a finding of habit, or regular usage, it would be admissible to aid the jury on its inquiry whether he did so on the occasion in question.

If that makes sense to you, I'll give you a doughnut. In the linked to article Alan W. Clark concludes:

[H]abit evidence, a form of circumstantial evidence, may be admissible in limited malpractice cases by proof of custom and practice to prove carelessness or carefulness of an act or occurrence when there is no recollection of the facts. Such evidence may be used to support or deny summary judgment to a party. However, circumstantial evidence is no substitute for medical expert opinion to prove the relevant standard of care and whether good and accepted medical practice was complied with or violated by the defendant(s).
Moreover, courts must consider whether such habit or circumstantial evidence belongs to the creative imagination of the party or attorney and is being used in such a way to unjustifiably excuse an act or omission of carelessness or negligence where the party has no recollection of the events. One can only imagine the unlimited circumstances where meritorious claims or defenses may be defeated by abuse of circumstantial evidence. Otherwise, we may one day be faced with evidence of a party's custom and practice to do the right thing and never be wrong, despite the facts of the case.