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In the past few years the number of well-sampled optical to NIR light curvesof long Gamma-Ray Bursts (GRBs) has greatly increased particularly due tosimultaneous multi-band imagers such as GROND. Combining these densely sampledground-based data sets with the Swift UVOT and XRT space observations unveils amuch more complex afterglow evolution than what was predicted by the mostcommonly invoked theoretical models. GRB 100814A represents a remarkableexample of these interesting well-sampled events, showing a prominent late-timerebrightening in the optical to NIR bands and a complex spectral evolution.This represents a unique laboratory to test the different afterglow emissionmodels. Here we study the nature of the complex afterglow emission of GRB100814A in the framework of different theoretical models. Moreover, we comparethe late-time chromatic rebrightening with those observed in other well-sampledlong GRBs. We analysed the optical and NIR observations obtained with theseven-channel Gamma-Ray burst Optical and Near-infrared Detector at the 2.2 mMPG/ESO telescope together with the X-ray and UV data detected by theinstruments onboard the Swift observatory. The broad-band afterglow evolution,achieved by constructing multi-instrument light curves and spectral energydistributions, will be discussed in the framework of different theoreticalmodels. We find that the standard models that describe the broad-band afterglowemission within the external shock scenario fail to describe the complexevolution of GRB 100814A, and therefore more complex scenarios must be invoked.[abridged]

Afterglow rebrightenings as a signature of a long-lasting central engine activity? The emblematic case of GRB 100814A