Question on an Issue with Stata

Hello,
The tab command gave wrong output ( I have been working on the data set for a long time and tabbed same variables many times before), especially, displayed an alphabet on tab frequency and for few more variables too, inconsistent values. I tried an regression that I have ran many times and it gave different estimates. I quit stata and restarted it and it started showing correct original values. Can someone comment on why this may have happened? Thanks and appreciate your guidance in this.