Basic sort, with annoying bug

Python 3, 36 bytes

Bug-free version, 37 bytes

lambda l:sorted(l,reverse=l[-9:]==[])

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Annoying version, 36 bytes

lambda l:sorted(l,reverse=l[9:]==[])

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This depends on the input and therefore does not qualify for the bonus.
It has a probability of around 2% to fail. It fails when the input length is lower then 10.

Combined with LyricLy's answer this gets 34 bytes:

lambda l:sorted(l)[::l[9:]>[]or 1]
lambda l:sorted(l)[::l[9:]>[]or-1]

05AB1E, 5*0.9 = 4.5 bytes

Working solution

{TLΩi

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Explanation

{      # sort input
 TL    # push the range [1 ... 10]
   Ω   # pick a random number in the range
    i  # if true (equal to 1), do nothing

Solution containing the bug

Gives the wrong solution 10% of the time (independent on input).

{TLΩiR

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Explanation

Same as the working solution, except it reverses the list if the number picked is true.

Jelly, 7 * (100% - 10%) = 6.3 bytes

Ṣ¹⁵X:¤¡

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Buggy version:

ṢẊ⁵X:¤¡

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In both links, there's a test harness that will run the code 100 times, each time with the list you give as the argument, and then return the results.

The probability of each input length is:

0.1 - 0.1/(length!)

So for length 1 there's a 0% probability, for length 2 5%, for length 3 8.83̅%, for length 4 9.583̅% etc. until length ∞ which has a 10% probability.