If you have finished the amida's yihiyu l'ratzon(but have not yet finished the segment from Elohai netzur through u'chshanim kadmoniyot), you may reply to:

• Bar'chu,
• Kedusha,
• Amen to ha'el ha'kadosh,
• Amen to shomei'a tefila,
• Modim anachnu lach (just those 3 words),
• Birkat cohanim, and
• Kaddish.

However, you may not say anything else (such as other amens) until you have completed the entire amida (including the last word, kadmoniot).
 

Examples

• You said the weekday version on Shabbat or Jewish festival--or the reverse.
• You began to say the version for a different Shabbat service (say, it is Shabbat mincha and you said the version for Shabbat shacharit).

As on Shabbat, the minimum total volume of Jewish festival kiddush beverage that must be drunk--usually by the kiddush-maker (mevareich) but it may even be several people combined--is a cheekful (m'lo lugmov)—considered to be 2 fl. oz. (59 ml) within 30 seconds.

If you fly through Shabbat (permitted for business only), you must keep even Shabbat d'rabanan while flying over the area of the earth that is in Shabbat.
If you fly east (such as from Asia to the US, across the Pacific) after Shabbat is over, you will re-enter Shabbat and may not do melacha on the airplane. Say the Shabbat prayers and kiddush at the appropriate local time where you are flying.
 

You do not need to relight Chanuka candles that blew out after having burned for less than 30 minutes after dark, if they had been lit in a place where they would not be expected to have blown out.
If they were lit in a windy or drafty place, you must relight them in a different place and say the blessings again.
 

To avoid making a blessing in vain (bracha l'vatala), you may intend a fore-blessing to cover all other foods of that category that you will eat at the same time.

Don't fast on Shabbat (except Yom Kippur!) past halachic midday:     

• If you will not finish shacharit before halachic midday, you should eat or drink earlier in the day, even before you begin shacharit—water can be sufficient for this purpose.
• If you will finish shacharit but not musaf by halachic midday:

• Finish shacharit,
• Make kiddush,
• Eat some mezonot, and then
• Return to say musaf.

A cohen/priest does not need to see the congregants and they do not need to see him during birkat cohanim. Birkat cohanim is effective even if you are outside the synagogue when you hear it.  But if you are at the front of the synagogue such that you would be behind the cohanim when they turn to face the congregation and say the blessing, you should move far enough so the cohanim will be facing you during the blessing.
 

SITUATION It is Rosh Chodesh in a Jewish leap year.
WHAT TO DO
Add “ul'chaparat pasha” to musaf—from Rosh Chodesh Cheshvan through and including the second month of Adar. 

Since two days of Jewish festival are observed outside of Eretz Yisrael, read the lines for the sacrifice for both days that might have been the correct day.

Ideally, fulfill the commandment of a third meal (se'uda shlishit) by:

• Washing hands,
• Saying the ha'motzi blessing over two challot, and
• Eating at least the minimum amount (1.9 fl. oz., or 56 ml) of bread.  

You may, however, fulfill the requirements of se'uda shlishit by eating any solid food which gives nourishment—as long as you can say the after-blessing and have eaten at least 1.9 fl. oz. (56 ml) of that food.

(from kashrus/kk-medi-terumos.htm">http://www.star-k.org/kashrus/kk-medi-terumos.htm, used with permission)

Post this document in a conspicuous place.

1. Break or cut off more than one hundredth of the food and set it aside (for teruma and terumat ma'aser).
2. Say the following (either in Hebrew or English):
   
   
   
   Yoteir me'echad me'me'a she'yeish kahn harei hu teruma gedola be'tzad tzefono. Oto echad me'me'a she'yeish kahn ve'od tish'a chalakim k'moto be'tzad tzefono shel ha'peirot harei hu ma'aser rishon. Oto echad me'me'a she'asitiv ma'aser rishon asuy terumat ma'aser, uma'aser sheini b'dromo, u'mechulal hu ve'chumsho al peruta be'matbei'a sh'yichidita lechilul ma'aser sheini ve'revai. Ve'im tzarich ma'aser ani ye'hei ma'aser ani bi'dromo. Im hu revai ye'hei mechulal hu ve'chumsho al peruta be'matbei'a she'yichidita le'chilul ma'aser sheini ve'revai.
   
   (Im ma'aser minim harbei tzarich le'hosif) “kol min al mino.”
   
   
   Translation
   (If there is a food of one type that requires separation) Whatever is MORE than one hundredth of this food shall be teruma on the north side of the piece that I have set aside. The one hundredth that is left in the piece I have set aside plus nine other pieces the same size on the north side of the food shall be ma'aser rishon. That same one hundredth in the piece I set aside that I have made ma'aser shall be terumat ma'aser.
   Furthermore, I am proclaiming ma'aser sheini to be in effect on the south side of the food, and I am redeeming it and its fifth on a pruta (smallest amount of money recognized by the Torah for most purposes) of this coin which I have in front of me. If this food needs ma'aser ani, the ma'aser ani shall take effect on the south side of the food.
   If this food is subject to the laws of neta revai then it and its fifth shall be redeemed on a pruta of this coin that I have in front of me.
   
   If there is a food of more than one type, add each type of food for its type.
    
3. Wrap the broken or cut-off piece in plastic and discard.
4. The coin--dime or coin of greater value--must eventually be disposed of in such a manner that it will not be used.
5. The food may now be eaten.

 
If you do not want to say the long version, you may say this shorter version, after having separated a piece larger than 1\% of the total food:
All separations and redemptions shall take effect as is specified in this Star-K document outlining the Procedure for Separating Terumot and Ma'asrot, Tithes and Redemptions, which I have in my possession.

Whether saying the long or short version, only a little over one hundredth of the food will not be permitted to eat; all the rest may be eaten. Even though the tithes constitute over one fifth of the food, one is permitted to eat most of the tithes oneself, even though he may not be a Cohen or a Levi. Under no circumstances will it suffice merely to break off a piece of the food and throw it away. The aforementioned instructions must be strictly followed. The laws of the tithes apply to everyone, including the Cohen and Levi.

You must say the blessing lehafrish trumot u'ma'asrot if you know the produce definitely needed to have teruma and ma'aser taken; it was definitely:

• Grown on Jewish-owned land in halachic Eretz Yisrael and
• Had not yet had teruma and ma'aser taken from it.

BUT you must not say the blessing if the produce might have:

• Not been grown on Jewish-owned land in halachic Eretz Yisrael, OR
• Already had teruma and ma'aser separated.

You may not light Shabbat (or Jewish festival) candles before plag ha'mincha. The candles must burn until at least dark (tzeit ha'kochavim) and someone must be there to see the light from the candles after dark.
SITUATION Mincha minyan begins at plag ha'mincha. You cannot light candles at home and still get to mincha minyan on time.
WHAT TO DO You must say mincha on your own (anytime from half an hour after mid-day until sunset). You will light candles after plag ha'mincha but before sunset and not join the mincha minyan.

If you arrive late for a shacharit minyan, or if you are lagging behind the leader, you may skip all of psukei d'zimra to catch up--except for:

• Baruch she'amar,
• Ashrei, and
• Yishtabach.

You may also not skip any parts from yishtabach to the amida.