It can be. The really bad outcome is reverse-charging, where one cell is completely drained, then charged to a negative voltage. The probability of an explosion is significant.
If the flashlight has a low-voltage shutoff for the series at 2.5V per cell, it would take a series of four before that's possible, and even then it's pretty improbable. You'd have to mix a discharged cell with three full cells; a moderate difference in capacity or internal resistance wouldn't be enough by itself.
It is easily possible to over-discharge a cell under those conditions, after which charging and using it again is risky, but if you're salvaging laptop cells you probably already know that.
Thanks! I hadn't even considered the possibility of reverse charging really imbalanced cells. Not an issue for anything I use those recovered cells for, but certainly something I'll watch out for.
[this also reaffirms my decision to store lion batteries and their charger in a metal cabinet in a detached garage... because clearly I didn't know all the ways I could turn them into bombs.]
It's my opinion, having been using and reviewing Li-ion flashlights for several years now that single Li-ion cells are very easy to handle safely provided they and the devices you use them in are of reasonable quality. It's mostly a matter of avoiding short circuits and mechanical damage.
Multi-cell devices require a little more knowledge and care, but the main thing is just to make sure the cells match. Being the same model and charged to full before installation will do for new cells, but salvaged ones need to be tested and binned.
(though usually I use flashlights that take one 18650).