I'm using Particle Playground to generate a stream of particles that creates a line. I am using keyframes to move the cannon. The problem is that when it is rendering, particles are emitting from where the cannon will be one frame in the future. The problem seems to be worse when there are a lot of particles. Is this a bug, or am I using it wrong? For now, I guess I can just slide the keyframes one frame backward, but it doesn't seem like this is how it should work.
Does this happen for other people or just me?
There's nothing wrong. A particle cannon is a directed emitter and your settings clearly show that you are only using the default 10 deg spread, which will directly fire the particles forward in the direction the emitter is aiming plus your barrel radius is still 200 px, meaning your emitter is huge to begin with inside which to drop particles. Adjust your settings.
Maybe drop the velocity to zero as well? From my understanding velocity makes the particles fire out, where it seems like you just want them to be dropped, not fired. I might be wrong though