Up until a few years ago, when a player was folding their hand and it accidentally was exposed, I simply instructed my dealers to announce the mucked hand and move on; just like in a cash game. While participating in a large tournament event, the staff were issuing penalties for accidentally exposed cards. I had never seen this before and spoke to the staff about it. I don't know why it never really clicked with me before, but I agree with the discussion that took place. Essentially, why would it matter how a card was exposed (meaning intentionally or not)? An exposed hand could have a big impact on remaining action, therefore we should hold our players accountable for folding their cards not just "properly" but also face down.
Rule 60 currently states : "A player who exposes his cards with action pending may incur a penalty, but will not have a dead hand. The penalty will begin at the end of the hand. When folding, cards should be pushed forward low to the table, not deliberately exposed or tossed high (“helicoptered”). See also Rule 57."
What I propose is simply an addendum which clarifies that intentionally or unintentionally doesn't matter.
A player who intentionally or unintentionally exposes his cards with action pending, may incur a penalty, but will not have a dead hand. The penalty will begin at the end of the hand. When folding, cards should be pushed forward low to the table, not deliberately exposed or tossed high (“helicoptered”). See also Rule 57.