A prolonged bull market across stocks, bonds and credit has left a measure of average valuation at the highest since 1900, a condition that at some point is going to translate into pain for investors, according to Goldman Sachs Group Inc.

"It has seldom been the case that equities, bonds and credit have been similarly expensive at the same time, only in the Roaring '20s and the Golden '50s," Goldman Sachs International strategists including Christian Mueller-Glissman wrote in a note this week. "All good things must come to an end" and "there will be a bear market, eventually" they said.

As central banks cut back their quantitative easing, pushing up the premiums investors demand to hold longer-dated bonds, returns are "likely to be lower across assets" over the medium term, the analysts said. A second, less likely, scenario would involve "fast pain." Stock and bond valuations would both get hit, with the mix depending on whether the trigger involved a negative growth shock, or a growth shock alongside an inflation pick-up.