Several times in my own electronics and many times in secondhand items. The potassium hydroxide electrolyte never quite evaporates and seeps under the crevices, requiring a full disassembly to prevent further corrosion.

Worst offenders are the pesky little alkaline button cells. Every alkaline cell inevitably evolves hydrogen gas as the electrochemical reaction proceeds and in those tiny canisters, the gas very quickly has nowhere to go but out along with the cell's guts. The secret is to get the silver oxide versions, same voltage, a bit pricier, but far less leakage.

I guess the silver lining to the declining popularity of AA and AAA powered devices is that I can finally afford to use only Ni-MH or primary lithium cells in all of my devices. Those chemistries are mostly immune to leaking.