We have posted before about the relativity of technical "absolutes". While there are certainly some universal constants in lifting, throwing, or any physical skill; there are also times when an individual's particular strengths or proportions allow for, or even dictate, deviations from the standard ideal. Take this clip of Ilya Ilin for example. He appears to "loop" the bar slightly around his knees as he transitions from the first to the second pull. He definitely imparts some horizontal motion to the bar. The ideal is a smooth, straight, vertical pull with a slight rearward trajectory. Certainly no one intentionally teaches a horizontal swinging of the bar. Having said that, who would try to "correct" a lifter who can manhandle 240 kg. in this manner? Obviously Ilya has inhuman strength levels and this little "swing" works well for him. It's like the Dimas head snap at the finish of the pull. Certainly not text book and not what you would teach a new lifter. But who can argue with his success?
There are examples of lifters who pull with rounded backs or bent arms. They are rare and should not be copied, but for various reasons, these deviations work for them. Teach the rules, but be wise enough to allow for exceptions in some athletes.