Determining identity of unknown compound with NMR data
Peaks of acidic protons in an NMR spectrum can change (in position, form and intensity) due to exchange with other volatile protons.


A very fast exchange makes the peaks coalesce, you get one singlet between the two "original" (expected) chemical shifts, the exact position depends on the respective amount of both. 
A slower exchange, one "on the NMR timescale", broadens that peak considerably, typical case is residual water and a COH proton. The peak might in some cases be so broad it becomes invisible.
Even slower, this peak splits into two, which become sharper and further removed from each other.
A very slow exchange makes no change to the positions, but an exchange with deuterium (i.e. from the solvent) changes the relative intensity of the respective peaks over time.