Never say something can 'never happen'...it will.

I use foil tape in abundance on the outsides of glass and plastic enclosures alike. Mostly to tape the UTH on and the t-stat probe to the UTHs. Through this use I know two things about it:
It is VERY sticky with sharp edges.
It can roll up from the corners with a lot of movement and debris.

Now, I don't know about you...but my snakes love to move around at night. Any bedding you use can get onto the edges of the tape, causing the stickiness to reduce and the edges to roll up with movement. I would NOT want my snakes coming in contact with that stuff, since it's hard to get off my own skin.

I prefer being certain of safety over having to occasionally reposition a thermometer probe.