(1) Confusing open and closed pipe harmonics — closed pipe has ONLY odd harmonics. (2) Using $f = v/(2L)$ for a closed pipe (should be $v/(4L)$). (3) Forgetting end correction in resonance tube calculations. (4) Doppler sign errors — always ask "is frequency increasing or decreasing?" as a sanity check. (5) Confusing "overtone number" with "harmonic number" — in a closed pipe, the 1st overtone is the 3rd harmonic. (6) Assuming beats are possible for any frequency difference — beats are only audible for $\Delta f \leq 7$ Hz. (7) Not converting km/h to m/s in Doppler problems.