RowHammer
Beginner
A flaw where reading one row of DRAM over and over can flip bits in the rows next to it.
Novice
A DRAM disturbance error: activating the same row very many times between refreshes makes charge leak faster from cells in neighboring rows, flipping bits that were never accessed.
Expert
Caused by repeated word-line toggling coupling into adjacent cells. It became a security problem because software can trigger it. Mitigations: more frequent refresh, target-row refresh of neighbors, and access counting in the controller or the DRAM.