True examples are hard to find, obviously, because by definition I don't know the words I won't know.Sure, *you* don't know yet what words that you don't know you'll come across in the future.
But this ("offside", that is -- not "y'all") was aparently not just an example of *you, the recipient*, not getting it right.
I've run across it in the past, however, with regional differences in the US.What -- you've really come across cases where someone uses regional-dialect expressions that even *they, the speaker*, don't get right, often enough for it to be worth mentioning here?
And, to get back to the point: If you really have encountered *that*, are you seriously claiming that you *didn't* think that was "excessive" use of regional dialect (in the sense of hindering communication with out-of-regionals like you)?
I find it hard to believe that you really think the onus was on you to understand something that even the person who was saying it didn't understand. Is that really how you feel?