If you download your driver’s card after every shift you won’t leave your card in the tacho as you will notice that you don’t have your card when you go to put your card in the download machine.
When you get in tomorrow, do two printouts and make written entries on the back showing the correct end of shift and Rest times, with a short note stating that you accidentally left your card in the head unit. Sign both. Keep one with you for 28 days and hand the other one in.
Yes, you can drive as normal tomorrow.
No, you won’t get fined/imprisoned, but yes you will (probably) get an infringement letter from your employer.
You wont go to prison on this occasion; it’s a common mistake and experienced drivers are as guilty as rookies. When analysing the report, it’s blatantly obvious what has happened. That doesn’t stop it being wrong. It is certainly an infringement but try not to make a habit of it.
As stated earlier, make sure you take printouts and mark accordingly.
All the best, Pete 
Read this earlier then gone and done the same tonight. Although I’m sure I have ejected it.
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I got in and it was poking out the machine, yay!
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Done the same thing couple weeks ago. Just there was noone to take it out for me
;( just kept a note of my driving/breaks for that day and did printouts for them just in case 
although tacho kept screamming about insuficient daily rest for the whole day
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same thing happened to be OP, but i did something even more stupid i drove without the card in but im a agency driver and 1month has passed now so im in the clear i think.
But yes writing on the back of the printout is what your supposed to do but noobie dont know this
Sounds like you got a good shunter there! Most would tell you to beep off as they are busy…mostly sitting in corners of the yard.