I was sitting East here; pairs MPs, NS vul, dealer S:
North became declarer in 3♥ after W had opened and re-bid his diamonds. I was all poised to table my Q♦ when to my alarm I saw the A♦ face-up opposite! It is normal practice in our club to make the opening lead face-down and ask "any questions" as per Law 41A - I don't know how this went astray on this occasion but it's a pity, seeing as I could have alerted partner's out-of-turn lead before any harm done!
Anyway declarer chose to call the TD who explained the two options: either he could accept the lead, in which he had the choice that either he or his partner could be declarer - or he could refuse it, in which case the A♦ was a MPC. He understandably chose the latter, and then forbade me to lead a diamond. I accepted this in good grace of course: as far as I can recall this is the first time I've ever had a lead penalty imposed on me. I then had to ponder whether my partner had an entry elsewhere. By good fortune I chose to lead a club....
But it made no difference. West was able to cash his three winners but on the third diamond North could afford to ruff high, so I didn't get my trump trick. After losing the ♠ finesse he still landed his nine tricks safely enough.
I looked at the travellers and another table reached the same contract and made it - presumably without mishap! So I don't see how my partner's lead-out-of-turn made an atom of difference. Other tables bid different contracts including 4♥-1, so we didn't do too well on the board - but that's another story.
It is not always the case that infractions like this are benign, though. Perhaps others would like to contribute their own triumphs/disasters based on such an occurrence.