It happened to me several times on my history as a Pokemon player, but the first time I encountered it, I was really freaked out ! O_O
So, imagine the scene. I was at my grandparents'. Out in their backyard with my SP. I've been trying to defeat Steven ever since I woke up that morning. It was him vs. my Lv. 92 or something Blaziken. (I was 5 years old !)
Then, I chose Fight and I had no selection. My Blaziken used Struggle.
Well, back then, I had no idea what Ethers and Elixirs did. So I just sold them. I also had no idea that Leppa Berries existed. So, I went through the Elite spaming Blaze Kick and Sky Uppercut, leaving Rock Smash and Strength for Steven.
So, er-hem, I thought the game broke with Struggle, so I turned the GBA off. It was a wonder I knew how to save, but I was still stuck at Sydney. After somewhere between one week and a month, I managed to discover I needed Blaze Kick and Sky Uppercut for Steven.
I now giggle every now and then when I see Struggle appearing. It makes me think of what happened back then !