I sat performance and Mass and Balance at Silsoe during the week of November 4th last year. On each paper there was a typographical error that effectively rendered two questions unanswerable. The worst was on M & B; it was a very tight exam, time-wise, and this one question had a digit missing off the correct answer. I could see what had happened, but it still created enough doubt to make me spend another couple of minutes on the question than I had time for. After the exam, the invigilators were surrounded by a lynch mob, so it wasn't just me.
If you do get a dodgy question, do your best to memorise it's number and as much as you can of it, and tell your ground school ASAp, so they can query it. Questions are quite regularly disallowed, and everyone who sat the paper credited with the marks for that question