What does "Addition is NOT guaranteed to work" mean?

I'm new to Arduino and have just been reading through some threads in this forum. I found quite a few posts where someone says something like "Addition is NOT guaranteed to work" or "Subtraction is guaranteed but addition is not", however I can't find any details on what this actually refers to (on the surface, saying addition doesn't work sounds a bit crazy!).

Can anyone provide (or point me to) more info on exactly what is meant by this?

Thanks!

Someone helpfully sent me this link that explains in detail:

https://www.gammon.com.au/millis

It turns out not to be some general issue, but rather specifically to do with comparisons and overflows (which is common when people are using millis() to trigger something at a specific interval). Makes perfect sense now - thanks!

DanTup: Can anyone provide (or point me to) more info on exactly what is meant by this?

I guessed the context in which the comment was made (and you have already discovered it) but, for the future when you have questions like that it is a big help if you provide a link to the statement you are referring to. In another case it may not be so easy to guess.

...R

Robin2: for the future when you have questions like that it is a big help if you provide a link to the statement you are referring to. In another case it may not be so easy to guess.

Yep, that makes sense and I should've - I made the assumption that it was a general statement (since none of them qualified for that particular instance) but I now see that was incorrect. Thanks!