TLAs
I work for a big American Engineering organisation where entire conversations are conducted in TLAs. Sometimes it's OK if you are in the know but other times ... especially if I cross disciplines say from Aviation where I was to Rail where I is, the same TLA means something completely different.
Fortunately I am old enough, ugly enough and senior enough to stop the meeting to say ... kindly explain and stop trying to confuse me - and often my junior staff will confide afterwards that they didn't know either but were too embarrased to ask. How safe is that ???????
The real laugh is that very few people who use them have ever heard of a TLA. A couple of years ago I kept the in-house lawyer puzzled for days on that one.