  Radio variability in quasars has been seen on timescales ranging from days to
years due to both intrinsic and propagation induced effects. Although
separating the two is not always straight-forward, observations of singular
`events' in radio light curves have led to two compelling, and thus far
unresolved mysteries in propagation induced variability--- extreme scattering
events (ESE) that are a result of plasma lensing by AU-scale ionized structures
in the interstellar medium, and symmetric achromatic variability (SAV) that is
likely caused by gravitational lensing by $\gtrsim 10^3\,M_\odot$ objects.
Nearly all theoretical explanations describing these putative lenses have
remarkable astrophysical implications. In this chapter we introduce these
phenomena, state the unanswered questions and discuss avenues to answer them
with a $\sim $weekly-cadence flux-monitoring survey of $10^3-10^4$
flat-spectrum radio quasars with the ngVLA.
