Player plays before opponent decides on mulligan
This has in fact happened to me on multiple occasions.
I've gotten into the habit of never looking at my opening hand, when I'm going second, until my opponent declares whether they're keeping their opening hand or taking a mulligan. (This is to stop anyone who tries to read my reaction to my opening hand.)
However, sometimes my opponent won't wait for me to say whether I'm keeping or taking a mulligan, and will just make a land drop (and sometimes play something, too) and pass the turn as soon as they've said that they're keeping.
At Regular REL I would suppose the remedy is just for the opponent to take the cards back into hand and let me decide whether to keep or mulligan (and this has only happened to me at Regular REL). What would happen at Competitive REL?