Tanning Mom Patricia Krentcil has committed the cardinal sin of tan addicts -- stuffing her orange, leathery skin suit into a tube of fake UV light ... i.e. a TANNING BED!!!!

Our tanning sources tell us ... while in Palm Beach Gardens, Florida -- where she's staying for rehab (alcohol, not tanning) -- Patricia stopped by a Copper Tan USA to get her brown on ... despite vowing in the past​ to never fake tan again.

And get this ... we're told Patricia didn't buy just one session, she bought NINE. Here's the breakdown:

-- 8 sessions for $40 in a level-one bed (the lowest level of cancer light tanning avail)
-- 1 session for $32 in the strongest bed ... the "Celebrity Bed" ... which she USED the same day.

According to the receipt Patricia plunked down $113.10 (she also bought tanning lotion).

Cancer cells come free.