Yes, is true. The errors will increase when you pansharp raster layers that have bigger resolution differences (ex: IKONOS, QuickBird and WV2 they have 1:4 spatial resolution differences). For the Landsat 7/8 the ratio is smaller, 1:2 (15 m panchromatic band, 30 m multispectral) so erorrs should be smaller. It also depends on what pansharpening method you use. Try different techniques.
If you want to classify a pansharped image, a better alternative solution to the traditional per-pixel methods - supervised and nonsupervised (Maximum Likelehood, ISODATA, etc) would be an OBIA classification.
OBIA is less sensitive to false pixels because it first creates image objects (poligons) and then it classify these objects, rather than individual pixels. Theoretically, a pansharped image should produse better classification results when using OBIA.
eCognition and ENVI EX are the most common OBIA softwares.
Arhanghelul