I now test rcites for 4 configurations on Travis so Travis runs 4 jobs. For a while, I was running all the jobs at once, and as rcites is R-client for a web API, many of the jobs failed because of the well-known error 429:

Too Many Requests (RFC 6585) (HTTP 429).

I always ended restarting some jobs, I coped with it. At some point, I stepped back and thought: “people people behind Travis are smart, so they should have thought about limiting concurrent jobs”. You bet they did! It’s one of the many settings available. So I simply switched the button Limit concurrent jobs in Settings/General and on a set the limit to 1. No more HTTP 429!