Is stevens-johnson syndrome a side effect of ibuprofen?

Uncertain (too rare): Stevens johnson syndrome (toxic epidermal necrolysis) is a severe, sometimes fatal, allergic reaction. It occurs in about 2-4 people per 1 million people each year. About half of the cases have no known cause, more than a fourth of the cases are felt to be due to infections by viruses, mycoplasma, bacteria, etc... The remaining 20% or so of cases are believed related to a variety of medications.

Yes: Stevens-johnson's syndrome has been reported after use if ibuprofen. It is very rare however.