This isn't really magic but it's worth trying! It's an oldie but goodie - make one up for yourselves and see how many people get it wrong!
On a card, perhaps a blank playing card or smaller, print (with right justification so all the final zeroes are under each other, not as it looks below):
1000
20
30
1000
1030
1000
20
-------
4100
Put this in one of those small envelopes ready to whip out at any opportunity. When you select your mug, tell them that this is a simple test of addition, and all they have to do is add on numbers and tell you the answer. Take the card out, hold the envelope across it under the first figure of 1000 and ask spec to read it out. When they've said '1000' move the card down under the 20 and wait for them to say 1020. move the card down again and try to get the speed up a bit. When the envelope is under the final 20 there's every chance that they will say 5000 and, at that point, remove the envelope completely and show them the true total of 4100. They will probably grab the card out of your hand and furiously check it but the answer is correct!
Try it yourself and see.
That's just mean. (Grumble grumble.....)
(Dunno where I got that from!)