Possible spoiler: if you tip Moxxi $15k, she'll either give you Moxxi's Good Touch or Moxxi's Bad Touch. Does anyone know how many times you can tip her to get the weapon? FYI: I tipped her $15 at level 11 and got a level 11 Bad Touch, then I tipped her again at level 15 to get another level 11 Bad Touch. Now I'm over level 18 and I have tipper her over $60k and I still haven't gotten a new weapon from her. So based on my experience (on the Xbox 360), you can only use this twice, but I have read else where that there's no limit and the weapon scales to your level (which it did not for me on my second attempt).
You can get em infinitely but the amount you have to spend is totally random. It could be 100 or 1000000 but you should leave and enter the town after you get one so you don't get the bug seen in Schottlicious' answer.
EDIT: Read the answer by Terton. it's right! I experienced a bug in the game (thats my excuse^^).
I can't fully answer this, but I do know that my current level 50 has had his hands on multiple bad and good touches.
My hypothesis is that you can get both guns each playthrough (there are 2) and then once more after finishing the second. The do definitely scale to your level, my current one does about 5300 basic damage. By the way the bad touch is better than the good^^
Now recently I tried tipping her again but after 100,000 she said the giving gun dialogue without actually giving a gun. After that it became impossible to use the tip jar.
Hope this helps! EDIT: Read the answer by Terton. it's right!
You can get Moxxi's Good Touch and Bad Touch an infinite number of times, and, most importantly, an infinite number of times per playthrough. The people stating that you can only get Bad Touch once per playthrough are misinformed.
The amount of money it costs, as far as I can tell, only really applies for the first time you get it - noticeable by how every time you put a tip in the jar Moxxi will comment. After receiving the first gun, it then becomes a random chance per tip that she will comment and/or give you a gun. That's why it's suggested to only tip $100 at a time for the guns after the first since it will save a lot of money in the long run. As for selling/discarding the item, it should work the same; though I would suggest exiting to menu after selling/dropping it then going back in to be sure it's clear.
Also, there is no need to bother with the cumbersome two-player method to get more copies of Bad Touch. You can get Bad Touch again and again, solo, by simply transferring your character's copy(s) of the gun to another one of your characters via Claptrap's Secret Stash. As long as the gun is not present on your current in-game character (whether on-hand, in the bank, or sitting in the stash box) then Moxxi will continue to give you another one.
"Now recently I tried tipping her again but after 100,000 she said the giving gun dialogue without actually giving a gun. After that it became impossible to use the tip jar" I have had this problem on the ps3 borderlands 2. Try waiting a few minutes, walk around town open and close your equip menu and check on her again. Usually she gives the gun right away and sometimes she takes forever.
Also I tend to NOT tip her a second time after getting 1 gun during a visit to sanctuary as she does not seem to give a second weapon at all even if she says she will.
Meaning go there once, tip her, get your gun, fast travel somewhere else and then return if you want to try again. I believe the weapon does scale to your level, if you want better luck try tipping her in an online group of 4 preferably higher level players. I got my best one online.
I think I can answer some people's questions about these guns. I used the PC version and used cheat engine to spend like 10 million on testing this.
You can get this gun as many times as you want; just drop one of the two guns of hers you have, so that you only have one of her guns, (you can have only two) and quit.
Now re-enter the game and tip her until you get a new gun, this will be the second now and will be freshly levelled to your current level.
So if you have the cash you can get a fresh version of this gun every level!
Miss Moxxi's Good and Bad Touch can both be unlocked an infinite number of times for Xbox 360. The Good Touch you can get by tipping her yourself. The Bad Touch you can get by having a level 1 character join your game and tip her. The level of the weapon depends on the host. I am actually doing this right now. Specs for the best Bad Touch I have are
Apt Bad Touch Damage 4394 Accuracy 93.8 Fire Rate 9.0 Reload Speed 2.5 Magazine Size 32 Corrosive Dmg/Sec 4867.9 Chance to Corrode 18.1% (Based on Assassin at lvl 50)
Nope, all you have to do is save and quit. No matter how many good touches you have she'll keep giving them to you. Even if your save quiting and immediately going back to tip her.
Oh and it scales with the Main quest line your on currently. If your on a lv.20 main quest then your gun will be lv.20. If your on a lv.50 it'll be 50. So forth and so on. If you got power leveled by a friend and then start from the beginning the gun will be lv.1ish.