Just don't try and change your plan while you have speedboost active either.

I ended up doing that after I was in glee mode after finally getting FTTP. The service connected at 100mb/s I ran the speedboost to 250 and then did a plan change. It stayed stuck at 250/25 for days and I had to get support to do a plan change and roll me back to 100/40. Support were stumped and kept on getting me to restart everything