、。
If you lay it down on the desk it can roll off, so stand it up in the test tube rack.
Literal
Desk [genitive-の] top [at-に] lay-down-place [when-と] rolling fall [thing-that-happens-ことがある] [because-ので], test-tube-rack [in-に] stand-up-leave.
Another lab-procedure instruction. Two separate grammar points to catch. (1) 寝かせて置く uses the causative of 寝る ('sleep/lie down') — 寝かせる ('lay X down') — plus ~ておく ('do X and leave it that way'). Together: 'lay it down (in horizontal position) and leave it.' (2) ~ことがある attached to a dictionary-form verb means 'it sometimes happens that X,' 'there is a chance X' — a conditional possibility, not a habitual action. Distinguish from ~たことがある ('have done X') which uses the past form. The main clause ends in 立てておく ('stand it up and leave it'), mirroring the ~ておく pattern with the opposite orientation — you lay it down vs. you stand it up.