120 IBU is physically impossible anyway. Around 100 mg/L is the theoretical saturation limit for iso-alpha acids in a low gravity wort, and that is greatly reduced by both increasing gravity, and, counterintuitively, increasing hop rates. The most that will probably ever occur in finished beer is still shy of 80 IBU. High advertised IBUs are just machismo on the part of brewers. Ignore the numbers, brew the recipe, and see how you like it.
(Also note that the Tinseth and Rager calculators are capable of generating such an impossible number. But remember that these tables are really just their individual SWAGs at what they were getting on their own systems back in the day, extrapolated out to increasingly imaginary ranges. The fact that these tables have been canonized and enshrined in every book and software program published for a couple of decades tells us a lot about homebrewers' obsession with numbers, but very little about what's in our beer.)