If you find any mistakes in a course that is otherwise good, you can choose to ignore that particular word/phrase.
I’ve learnt to keep a dictionary handy when I first go through a course - not needed for everything, but very reassuring to check that you really are learning the correct spelling.
The worst bit for me is when you’re learning the same word on two different courses, but one has one variant and the other another, e.g. ysgubell and sgubell, or hyfryd and dymunol, or cynnes and twym. Very confusing when you’re at the Review stage and can’t remember which is the right translation for that particular course!
The best course I’ve come across so far is “Welsh Vocabulary (images and audio!)” which has over 1000 words/phrases and every single one has the audio with it, so no problems knowing how to pronounce them.