This one could get messy.

Yesterday afternoon, Qz reported on a message that some Netflix/Verizon customers were seeing which pinned potential video quality issues on Verizon’s network being “crowded.”

Verizon, as you might expect, was none too happy to hear about this — and now they’re threatening legal action.

The core of Verizon’s complaint: There are tons of points of failure along the road that could cause network slowdown, and if Netflix is going to point the finger at Verizon, it’s up to Netflix to prove it.

Message Screenshot below, via Yuri Victor on Twitter:

In a letter from Verizon addressed to Netflix’s legal counsel, Verizon gives Netflix five days to do the following:

Stop displaying any messages blaming Verizon for video issues

Provide Verizon with “any and all” evidence that Verizon was “solely attributable” to any video quality issues

Provide Verizon with a list of “all Netflix customers on the Verizon network to whom Netflix has delivered such messages”

If Netflix decides to comply with Verizon’s demands, that last one could prove hard to fulfill. Depending on how the error message is triggered (a quick client-side packet test, for example?), it’s feasible that Netflix isn’t actually keeping track of who sees it.