Never Ever put your water bottle on your desk in the actual exam.
So recently I gave the CDS exam and just behind my bench,some girl was sitting and having the water bottle on her bench and while marking her OMR,she accidentally pushed her bottle and spilled the water on her OMR sheet.
Her whole Sheet was tarnished,All the ink made OMR incomprehensible, She was literally begging for Another OMR sheet to invigilators but all efforts went in vain,She started to weep and As an aspirant I felt very bad for her that a small accident ruined her attempt,She didn't even come back to appear in 2nd(GS) Paper.
So My advice to all aspirants would be to close the lid as hard as you can each and every time you sip the water and put the bottle on the floor besides your sitting bench.