First, the golden rule: The whole point of DMing is to entertain the players. And yourself, too, of course. Don't make the common mistake of viewing the players as your opponents, and don't value the rules, your plan, or anything else over the player's enjoyment. If you need to bend a rule, give the enemies more (or less!) health, throw in a deus ex machina, make stuff up, etc., then do so; be flexible.

I've had a few bad DMs, and they were always bad because they valued the system's rules too much. They were obsessed with little nitpicky things, always looked up the exact rule for every single event even when it had a huge negative impact on the game's pacing.

Once, one of those DMs had us fighting in a misty forest. We were fighting a bunch of kobolds, but we couldn't see them in the forest. The fight dragged on for hours, and we reached a point where we were clearly and vocally not having fun with it anymore. At all. And yet he kept having new enemy reinforcements come in from the mists to prolong this fight. At the end, I questioned why he kept having new enemies enter the fight, and his response was that that were always there, and we just hadn't seen them. He had no choice but to have them continue coming out of the mists. He couldn't just make them disappear; it would be messing with his plans, and it would throw off the balancing of the exp.

Which utterly flabbergasted me. He's the dungeon master. If there is an enemy that we have not seen, it is the easiest thing in the world to adjust reality so that it never existed, if it's clear that it's existence is only going to further ruin the player's enjoyment. So this is an example of what NOT to do. You MUST be flexible and attentive to your player's moods.

Of course, different players expect different things from the DM. Some groups don't mind obvious deus ex machinas saving them. Some definitely do mind. I can't speak for your group.