A.  The most common problem with map-side joins is introducing a high level of code complexity. This complexity has several downsides: increased risk of bugs and performance degradation. Developers are cautioned to rarely use map-side joins.
B.  The most common problem with map-side joins is lack of the avaialble map slots since map-side joins require a lot of mappers.
C.  The most common problems with map-side joins are out of memory exceptions on slave nodes.
D.  The most common problem with map-side join is not clearly specifying primary index in the join. This can lead to very slow performance on large datasets.

Answer: C