I too suspect one of two things.
Either the cleaning was not good enough,, There are several cleaners.. but the number of cleaners that will remove EVERYTHING,,,, is precisely zero.. This is why when I made a mirror for an expierment in Physics class I used a series of different cleaners, Alcohol, Acid, and base, Between the three of them you can get glass so clean that water WILL NOT BEAD on the top of the glass (What makes water bead is oil by the way)... But no one cleaner will do the job. Not even acetone.
Suspect #2 is the water you dammed.. Some of it may have gotten past the dam or more water may have condensed before you applied the tape.