On BIG-IP 11.5.1-11.6.4, iRules performing HTTP header manipulation may cause an interruption to service when processing traffic handled by a Virtual Server with an associated HTTP profile, in specific circumstances, when the requests do not strictly conform to RFCs.
https://support.f5.com/csp/article/K19501795?utm_source=f5support&%3Butm_medium=RSS