Deadly sin is the wrong notes

Definition of "wrong notes": simply creating the note that is different from what is supposed to be the "right note". Narrowly defined, it means only about the key, but broadly defined, it would encompass intensity and tone or tonal quality.
Because each note (or tone in broad sense) is an indispensable component of musical performance, the wrong note is in every sense the things that should be avoided first and foremost, and failing to do that is the deadliest sin of all.

In order to avoid wrong notes, we must first nail down their causes. They are:
1. the performer does not have a clear understanding and thus picture of the right note
2. the performer fails to pay attention to every note
3. the performer left it to chance whether his or her touch would create the right note or not (probabilstic or gamble-like playing, I dare call it)

Countermeasures woule be:
1. full grasp of the positioning and meaning of each as a vital component of the whole through rigorous analyses and imaginations
2. on top of 1., pay particular attention in parctice and public performance as well to the priority notes and those who tend to lose attention by the performer. It requires not only concentration but also the flexibility and independence of all fingers so that the preceding finger motion does not constrain the suceeding motion and other physical aspects as well. Fundamentals of scales and arpeggios are indispensable to automate the group of notes played correctly through persistent and stedy practice would help this dimension a lot
3. in particular for those notes in sequence that require travelling distance, correct use of arms and body would also be needed in a coordinated way, let alone the travel needs to be the one with shortest distance without any abrupt and discontinuous motion, and placing the fingers just a split second before they touch the keys

... not every one of them is easier said than done, but the whole boils down to creating the beauty of sounds and any aspiring pianist, amateurs and professionals alike, should pursue them at all cost.