I would definitely NOT drink any alcohol when you've just stopped methadone. But not because I believe that the fact that you took 2mg methadone yesterday is going to put you at increased risk for overdose though - I don't personally think the risk in that regard should be significantly higher than if you drank the same amount of beer after the methadone was completely out of your system, it's just 2mg taken by someone who has been on methadone for months and whose tolerance was presumably much higher. Of course one should always err on the side of caution since we're all different and if you are going to drink be very careful with how much you are drinking and drink very slowly, especially if you haven't had alcohol for a long time.
My main reason for suggesting avoiding alcohol is because it could easily make you feel miserable. Alcohol makes methadone leave your system faster, and speeds up and intensifies withdrawal symptoms. I also found that tapering methadone has made me way more sensitive to negative side effects of alcohol, hangovers, etc. Whenever I drink I totally regret it as it makes me feel like crap.
The second reason is that as others said, I would avoid alcohol for quite a while since you are very vulnerable right now and it could be easy to pick up an alcohol addiction.
The third reason is that it could put you at increased risk for relapse: if you drink, then feel awful or feel the withdrawals more severely, it could really make you want to take some of your old drug of choice to feel better. Our decision making processes are impaired when under the influence of alcohol as well.