Everything was looking fine while trying to revive a single 3.7V Li-ion cell with my probably safe(*) method. Cell and voltage regulator were warm to touch, but it's also pretty cold in here these days (20°C in the room). When I disconnected the constant voltage source the measured voltage was 3.6V or so, showing a revived cell. My "procedure" calls for further checks of the open-circuit voltage,whenever I pass by the shack.
Thirty minutes after removing the charging current ... voltage across the cell was 0V and it was warm! Obviously it was self-discharging fast, producing heat. I moved the battery outside on a flameproof surface and let it exhaust the charge before hitting (gently!) the battery recycle bin.
(*) Feeding the cell with 4.0x Volts for 15-30 minutes while monitoring the current consumption to be (well) below 1 Ampere. This procedure should not (further) damage the cell and/or risk to set it on fire. YMMV.