Joint is acceptable. What was probably not acceptable was the way you connected it to another word. For example I was playing an opponent and I played PANDY (a Welsh textile miller). Crossing the A in PANDY was QAT. My opponent then complained that she couldn't play DEEP. So I asked her where she was trying to play it and she said she was trying to attach it to PANDY. Well that would've resulted in her either playing DEEPANDY (not a word) or DEEP/EQ (the latter being an abbreviation and not a valid play).
So what you have to remember is that the tiles you play must be valid words in all directions.