Note: this is all in Europe.
I am in the process of applying for a phd position. I think I have a decent chance (albeit still small of course) of getting the position, since it matches my profile and I know one of the supervisors well.
In 2015 I took a semester abroad, doing a minor in a different subject than I graduated in. This subject is not related to the phd subject. However, it was still a valuable experience (living abroad, being independent), which I think will help my chances slightly.
The problem is that due to an issue between the two universities, the study credits I got there ended up not being accepted, and I had to take extra courses at my home uni to compensate for this. Of course, this is ridiculous, because I basically spent half a year there for nothing (in terms of ECTS). I do have a signed document stating that I completed my semester there, as well as grade lists, but I don't have any official records saying I completed my courses.
Should I still mention the courses I followed abroad, with the risk of having to explain that I don't have the credits for it at a later stage? I may end up looking chaotic/messy if they find out I spent a semester in another university without officially passing any courses. However, not mentioning it leaves a half year gap on my CV and I also miss the independence/international experience checkmark. I did spend half a year in another country on another occasion, but that was work-related, not academic.
TLDR: took semester abroad, records of it are lost, no official proof I completed the courses. Do I mention this semester at all on my phd application? Courses are only very slightly related to phd subject.