No lasting damage is done by this, after reentering the character sheet speed is set back to what it was last before setting it to -1.

Reproduction:

-If you click your speed, go to customize, enter a negative number into any speed. When you click away from the customize menu, the screen will transition to the "We rolled a 1 on our API check". (in replicating this bug, the error code is not consistent, included one of them in case it helps)

-Sometimes when you enter negative speed and didnt leave the customize menu, the speed would automatically be set to 0 if it was negative, but that didn't happen very consistently.

-Entering the number from the keyboard and exiting customize immediately (by clicking on a differnt part of the screen); or using the triangles to lower speed below 0 consistently caused the error screen.

-Before the error screen shows up there is at least a couple of frames of a negative speed are displayed before the error screen appears.

The error seems to not be triggerable from speed reduction modifiers (as it defaults to 0) and only happens when you try to manually override the speed on the sheet with a negative speed.

The solution seems to then just be don't put in a manual override for negative speed. There's no reason to ever do that, since there's no such thing as negative speed (despite what Reverse Flash says).

I don't want to, I wanted to reduce my speed but accidentally set it to 0 before lowering it with the arrow and was surprised that it allowed it. So I did a small bit of testing and posted a bug report in case it's an easy fix.