… your shoulders won’t straighten anymore because you’ve been in a hunched-over position for so long
… your jaw starts hurting because you yawn so much
… your eyes start hurting because it’s gotten dark and you haven’t noticed, thus not turning on a light
… you turn on the coffeemaker without placing a cup underneath/filling it with coffee/filling it with water
… you start using the keyboard shortcuts of your translation program while writing an e-mail
… you can’t feel your legs anymore
… you spend more time correcting your spelling mistakes than actually translating
… you keep rewording your sentences in normal conversation – without noticing it
… you (simultaneously) interpret the news or conversations going on around you in your head
… you forget to eat and wonder if that gnawing feeling in your midsection might be the onset of a flu
… you can’t answer the phone without keeping tapping away at the keyboard
… your dog has to remind you to take a break (using more or less drastic measures)
… you’re surprised that your significant other is “already” home, when he/she is actually an hour (or more) late
… your time-tracking software asks if you are still working on the project since there has been no keyboard activity in 15 minutes – while you’ve been lost in thought
… your coffee is cold and you don’t even notice it
… there are no more clean dishes because you haven’t taken the time to load the dishwasher even once in a week
… you keep staring at the same sentence/paragraph/page without having a clue how to start translating it
…

These are in no particular order, but sadly, I have experienced them all.
Feel free to add to the list!