I know alternative splicing is possible in different cell types of an organism, or within the same cell at different developmental stages. There are several examples like this. But are there any instances of alternative splicing within the same cell at the same time? Perhaps some melanin variants producing shades of different color for example. Normal cells, not cancerous.
In complex eukaryotes like humans, alternative splicing is the rule, rather than the exception.
Eukaryotic splicing is managed by a complex regulatory system, including more than 100 different elements, some of which are enhancers and regulators. Thus, while one form will often be dominant, one should generally expect at least some alternatives to be present in a cell.
While the amount of work on population-level heterogeneity is still much larger than that on variation within single cells, studies of splicing heterogeneity have identified that multiple forms are often present simultaneously within a single cell. For example, in this recent study, Figure 2 shows that although the distribution between two splicing alternatives is bimodal, single cells exist with the full range of fractional mixes between the two forms.
I do not recall any cases of such behaviour for normal cells, off the top of my head but it must exist. Probably, alternative splicing in the same normal cell can happen in early embryo differentiation or in the case of protein localization of a specific peptide in different compartments of a cell.