On Saturday, March 11, 2017 at 10:14:11 AM UTC-5, Robin Harms Oredsson 
> The only pattern I've really found seems to be:
> - It always happens on aborts
> - It seems to be during some  server load
> - It only seems to happen during requests that only read data

I don't use ZODB, but I may have an idea on where to look / possible ways 
to create a repeatable test-case.

I think I've seen similar situations caused by transaction deadlocks or 
timeouts on readers high-concurrency situations when there is an extended 
write going on.  

for example, there are 50 connections reading and 1 connection writing. 
 sometimes the writer would grab a table/row lock for too long, which would 
cause a pileup of locks on the readers and a few would eventually fail. 
 under pyramid, the finished callbacks unconditionally fire after a request 
-- including timeouts and errors where there was never a properly setup 
transaction or database to cleanup.  So it would try to abort, but it never 
got started in the first place. 

this may be related to what you're experiencing.

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