Here's the log output of the above iRule when run on the two different versions:
Output of log when running through iRule on 10.1.0 3900 LTM:
Rule connection-limit-rule : Setting getHistory with 10.1.210.253_7039861 and 1271206346
Rule connection-limit-rule : Arrary set with 10.1.210.253_7039861 1271206346
Rule connection-limit-rule : Checking 10.1.210.253_7039861 for time 10.1.210.253_7039861
Rule connection-limit-rule : Setting getHistory with 10.1.210.253_8958191 and 1271206346
Rule connection-limit-rule : Arrary set with 10.1.210.253_8958191 1271206346 10.1.210.253_7039861 1271206346
Rule connection-limit-rule : Checking 10.1.210.253_8958191 for time 10.1.210.253_8958191
Rule connection-limit-rule : Checking 1271206346 for time 1271206346
01220001:3: TCL error: siebel-connection-limit-rule - can't unset "::getHistory(1271206346)": no such element in array while executing "unset ::getHistory($sbrequestID)" ("foreach" body line 7) invoked from within "foreach { sbrequestID requestTime } [array get ::getHistory ${client_ip}*] { log local0. "Checking $sbrequestID for time $requestTime" ..."
Output of log when running through same iRule on 10.0.1 3400
Rule connection-limit-rule : Setting getHistory with 10.1.102.108_924115 and 1271206504
Rule connection-limit-rule : Arrary set with 10.1.102.108_924115 1271206504
Rule connection-limit-rule : Checking 10.1.102.108_924115 for time 1271206504
Rule connection-limit-rule : Setting getHistory with 10.1.102.108_1614654 and 1271206506
Rule connection-limit-rule : Arrary set with 10.1.102.108_924115 1271206504 10.1.102.108_1614654 1271206506
Rule connection-limit-rule : Checking 10.1.102.108_924115 for time 1271206504
Rule connection-limit-rule : Checking 10.1.102.108_1614654 for time 1271206506
Rule connection-limit-rule : Setting getHistory with 10.1.102.108_7492669 and 1271206506
Rule connection-limit-rule : Arrary set with 10.1.102.108_924115 1271206504 10.1.102.108_7492669 1271206506 10.1.102.108_1614654 1271206506