In this article we propose two simple a posteriori error estimators for solving second order elliptic problems using adaptive isogeometric analysis. The idea is based on a Serendipity11According to Wikipedia: Serendipity means a “fortunate happenstance” or “pleasant surprise”. It was coined by Horace Walpole in 1754. One aspect of Walpole’s original definition of Serendipity is the need for an individual to be “sagacious” enough to link together apparently innocuous facts in order to come to a valuable conclusion. We feel that this applies for the present discovery, but it is of course up to the readers to judge.pairing of discrete approximation spaces Shp,k(M)–Shp+1,k+1(M), where the space Shp+1,k+1(M) is considered as an enrichment of the original basis of Shp,k(M) by means of the k-refinement, a typical unique feature available in isogeometric analysis. The space Shp+1,k+1(M) is used to obtain a higher order accurate isogeometric finite element approximation and using this approximation we propose two simple a posteriori error estimators. The proposed a posteriori error based adaptive h-refinement methodology using LR B-splines is tested on classical elliptic benchmark problems. The numerical tests illustrate the optimal convergence rates obtained for the unknown, as well as the effectiveness of the proposed error estimators.