Not Taking RMDs
Once you turn 70 ½, you have to take RMDs (Required Minimum Distributions) from 401(k) s and traditional IRAs. If you didn’t take any RMD, you need to file Form 5329 in which you explain what happened and how you tried to make things right.
Considering that it’s the first time you take required distributions, things will be sorted out pretty quickly. However, if you do nothing about it, there will be a 50% tax on the amount that should’ve been withdrawn.