Bees can starve in summer and winter
Emergency feeding is needed when a colony of bees is so short of food it is unlikely to survive for many days and starvation is the likely outcome if the colony isn't fed. Food shortage and starvation are not just winter problems, they are summer problems as well. It is likely to be the fault of the beekeeper in the summer, because checking for food should be one of the main points of inspecting. The winter is slightly different, because there are times when the hive will feel heavy, but the bees can still starve with what appears to be plenty of food. These could be isolation starvation, where the bees have run out of food on the frames they are clustering on and it is too cold for them to break cluster to move onto other frames, or the stores are granulated, so the bees can't use it.
In my experience, different bees behave differently when they are short of food, so the signs will vary. Prolific bees seem to carry on producing brood until they reach starvation. I'm guessing it's because they may have a high percentage of genes from bees that have evolved where the climate is such that starvation is unlikely. I find that with non-prolific bees the queen will usually slow down egg production, then go off lay, to conserve food. Sealed brood will often be uncapped and the contents chewed into little pieces, presumably to access any sugars.
What must be understood is the bees are short of food because the store they would have in a natural nest is not there and they are unable to forage for a variety of reasons. The adult bees will start to slow down and eventually get to the point where they are hardly moving. This takes a couple of days and at this point some will be dead, but with careful treatment the colony may recover.
There will be a different approach needed depending on the time of year and whether syrup can be fed.
If the weather is cold the bees will cluster tightly, possibly some distance from the frame top bars. If it is warm they may partly break cluster and be close to the top bars. Any food must be in contact with the bees, otherwise they won't find it.
Frames of food from other colonies can be put on the edges of the cluster. If you have candy, then force some down between the frames, so it is in contact with the bees. A block or pack can be placed on top of the frames, but with a continuous trail of candy to the bees. Put an eke or empty super on the brood box, then fill the gap with some form of insulation. Screwed up newspaper is good for this.
Once the bees are flying and colony inspections are possible any food shortage or starvation is likely to be the fault of the beekeeper. Colonies use a lot of food in the spring compared to what they use during the winter and stores will be depleted quickly. Starvation and food shortage in the summer is more likely with prolific bees, where some, e.g. Italian types, simply turn food into brood, whether there is any income or not. These are particularly vulnerable when the oil seed rape crop is taken off. If there is little or no food in the brood box and the weather is poor for a few days, starvation is a distinct possibility.
If the colony is very short of food they won't have any in the supers. You can either remove the supers, or leave them and feed through them.
If the bees are starting to slow down they won't be able to access feeders and will need dealing with straight away, but you need to do a bit of thinking about what is happening. I give training workshops, where I ask questions of the attendees and this is one of the topics I deal with. Many don't understand the situation they will be faced with and I think it should be part of beekeeping teaching.
If a colony is in this state I suggest the following procedure:-
If one colony is short of liquid food in the summer, there is a chance they all will be. This will probably mean there is a pollen shortage too.