Our family has always abbreviated the first name as we all only use our middle names, as in D. Mike Dewey. If reservations are under D. Mike Dewey and the passport says Dennis Mike Dewey - will this cause a problem?
For flights, it could be problematic and would depend on where you are flying to and from, your citizenship, and the experience and mood of the staff checking you in.
In such cases, the airline staff that check you in have a certain amount of discretion and for some journeys, they may well allow it, just as they would allow a case of first name, initial, last name.
In other cases, some airlines may seize on the opportunity to levy an additional charge at checkin to change the name to match. I have had this before when 2 letters in my name were transposed. A US$250 typo!
Many airports now require self check-in. A machine is almost certainly not going to allow you to pass without intervention from an airline representative.
One specific problem situation may arise if you are not a U.S. citizen and are attempting to board a flight to the U.S. In this case, it is unlikely that any airline would take the risk of allowing you to board. The reason being the huge amount of passenger data that the U.S. government requires, and the amount of checks and pre-processing that is done. If the airline perceives any risk that a discrepancy could cause you to be denied entry to the U.S. (or any other country for that matter), then they will not allow you to board.
Almost every advice you are likely to receive is to ensure that the name printed on your ticket matches the name of our passport.