I am on Challenge 16 where I am being asked to say, “I met someone who told me…”
I said, "Wnes gwrdd â rhywun ddwedodd wrtha’i…
But Cat said, I think, “Wnes gwrdd â rhywun wnaeth ddweud wrtha’i…”
I can see how this would work if I heard it correctly.
Is it just a variation (“who did say” instead of “who said”)? It just seemed to be dropped in with no warning.
Diolch yn fawr
Gruff
Yes… 
Thanks very much for confirming.
I am enjoying the course greatly.
Delighted to hear it’s going well for you so far!
Da iawn, Gruffydd - you’re going well there!
We do occasionally drop suff in without warning. It confuses you slightly, but prepares you even more for the “eh?” moments of a genuine conversation, and gives you some flexibility as well. We try not to confuse too much though!
I know it’s an old topic but I just want to say it was a relief finding it. I’d been trying to figure out what the pattern was that caused ddudodd to sometimes become something else in challenge 16. If either one works fine then I don’t need to worry. 
This completely threw me last night, to the extent I went back through the vocab lists of the previous 3 or 4 challenges totally perplexed. Couldn’t find it and as with most of the things I don’t entirely understand I just go with what the course wants me to do.
Good to come across this topic though and learn it’s nothing to worry about.
I just did exactly. Thank heavens for the forum!
I’ve just been baffled with this too- but looks like our way of translating it is just different but still correct? C’mon now after challenge 14 we don’t want any nasties! 