Certain petrochemical barrelling recipes have a chance to fail/output byproducts, leading to silly infinite barrel loops.
Noticed it with the following 'fill barrel' recipes so far:

• Petroleum gas
• Light oil
• Heavy oil
• Lubricant
• Sulfuric acid

Fixed!

I couldn't reproduce this at first, until I noticed that Scrappy Industry (not Scrappy Chemistry) was putting a stop to it. I copied the restriction over and everything works fine now.

I noticed this was still happening after I re-enabled the rest of my mods. After a bit of testing, it seems like running with Barreling Group prevents Scrappy Chemistry's restriction from working. Though I'm not sure if this is an issue on this end, or theirs.

Fixed in 0.6.2.

That mod changes the groups so technically speaking it is probably the other mod's problem but it was simple enough to fix that I went ahead and did so.