Many times when someone manually schedules a job to execute it will end up executing multiple builds of jobs that are in the same Category (limited to 1 per node and 1 in total) concurrently.
I have a single Jenkins server, no slaves, and a setup like this:
Job A - triggered by Git updates, part of category X
Job B - triggered by A on success, part of category X
Job C,D - triggered by A on success, part of category Y
Job E - triggered by timer, part of category X
When Git updates trigger Job A and while A is running I manually schedule Job E to run (both are part of category X that allows only 1 concurrent build), then Job E waits for A to finish as expected. But when A is done successfully then Jobs B, C, D and E start all at the same time.
Problem is the B and E are part of category X as well so they should never run at the same time.
I didn't see this before recently upgrading all plugins and Jenkins.