Widebody; Low Wing; 4 Turbojet Eng
Communication Breakdown; Confusion; Situational Awareness; Distraction
ATC Issue All Types; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Provided Assistance; Flight Crew Returned To Clearance
Human Factors; Procedure
We were cleared for the ZEEKK1 arrival expecting the ILS 26R approach into IAH.  This arrival has strict airspeed and altitude constraints at different waypoints along the procedure.  Because of the dissimilar aircraft on the arrival ([heavy] and smaller aircraft) ATC had us and the other aircraft fly airspeeds different than those published; everyone had a different speed. This created a lot of extra communications on a very crowded frequency.  Then ATC switched runways for us to 26L and tells us to resume published speeds as per the procedure.  It's very busy now as we set up for the different runway; manage the speeds and get a frequency change.  Once checked in; this controller says to expect 26R!  We told him the previous controller said 26L. He states 'I don't know why he said that but I need you on the right'. So again we switched everything back to 26R.  Very quickly he clears us to ZAPPO (the IAF for 26R) maintain 6;000 and intercept the LOC.  The read back did not include 'maintain 6;000' ATC didn't catch it; nor did I.  We were task saturated at this point as we switched RWY's again; trying honor the arrival constraints and follow ATC instructions. ZAPPO is 5000A so I put 5;000 in the MCP window and then started the descent. Just prior to the fix; the controller stated' I didn't clear you to 5;000; told you to maintain 6;000; It's OK there's no conflict; cleared the approach contact tower.'  The approach and landing were uneventful; no further communications from ATC about this.
A pilot reported about runway changes and confusion flying into IAH. Various runway changes lead to confusion and an altitude deviation. The pilot was told to maintain 6;000 feet on the approach but descended to 5;000 feet for a crossing restriction. The rest of the approach and landing were uneventful.