These days, DSLRs are powered by dedicated Li-Ion batteries, but what about flashguns, lightmeters and flash triggers? Many of these devices rely on AA or AAA batteries for their power, and leaving the cells in for extended periods can cause major leakage issues over time.
This poor speedlite had been left for a couple of months and the batteries had leaked rather badly. In this case, it was matter of carefully cleaning the contacts to remove the salty deposits and it was as good as new.
In many cases though, if the batteries have leaked excessively, the battery acid can corrode the contacts and find its way into the circuitry, rendering the equipment irrepairable.
So make a point of taking the batteries out and storing them in a separate pouch. If they leak, at least they won’t damage anything expensive!