Strictly speaking, Yeah’s analysis, above, doesn’t quite fit the situation. We are dealing with monthly deductions, not yearly tax (and I fully agree with Yeah when considering total yearly tax).
One factor that comes into play is whether OT pay is flat-taxed or subject to standard withholding. Since the calculation of standard withholding can become a nightmare, especially if the OT check comes at a different time during the month than the regular check (this is how it works where I work), it is often easier to subject the OT pay to a flat-tax based upon the current tax bracket.
Using a flat-tax scenario, consider the following:
Last month, your taxable income was $2392 - the max monthly amount for the %15 bracket. You FWT (assuming no deductions) would be about $325.65, and your ‘net’ (actually, for our purposes here, the Gross minus the FWT) would be $2066.35.
This month, say you got a $1/month raise. This puts you at $2393, which is the minimum for the 28% bracket (actually, it’s $2392.01). This would put your FWT at $325.93 and your gross at $2067.07.
So, without OT, your net increases slightly, as one might expect.
Now, let’s factor in 9 hours of OT:
Last month, you made $2392 regular, which would put your hourly rate at about $13.80 / hr. So 9 hours of OT grosses $124.20. Now, if OT is flat-taxed, they simply take out a straight percentage - in this case 15% (of course, this neglects the fact that the OT alone would actually put you over the edge in this case). So, the FWT is $18.63 for the OT portion, and the OT net is $105.57 (again, neglecting other deductions). Total monthly check, therefore, is $2171.92
This month, you now make 2393 regular, which translates to essentially the same /hr figure. However, your flat-tax rate is now 28%. So, you make $1 more regular, but you wind up losing 13% on the OT. So now, the FWT on the OT is $34.78, resulting in a gross OT of $89.42, and a total monthly net of $2155.77, or a loss of $15.16 over last month, despite a $1 raise!
Again, there are lots of assumptions being made here, and some simplified calculations, but my point is that it can happen that a raise might result in a lower net pay without any chicanery on the part of HR.