At Guademy I live translated three conferences from English to Spanish. I have never done that but, given that the translators we had hired told the organization the day before the event they would not come (very unprofessional, btw), there was no other option than using me and my broken English.
All in all, it was an interesting experience. I was told my translation was about 85% accurate, which is IMHO quite good: I had not trained for that, therefore my “buffer” was quite small. The most difficult part of live translating was a) lisp (I’m used to American English, not to British English) and b) memory (I seldomly need to memorize something, therefore my short-term memory is quite bad). Fortunately all the conferences were about open source software I know quite well, therefore specific vocabulary was not difficult.