Yep, I’ve done it accidentally before. I replied to what I thought was a mastodon account but it was a Lemmy sub. All the comments on that thread come to me on mastodon as replies.
Since I did it by accident, and they also didn’t know it, we were all very confused for a while.
I'll sometimes tag Lemmy communities in my mastodon posts. The only thing I dislike about it is how Lemmy displays tags in the post title. There's got to be some way to fix it so it's not so off-putting.